SCOTTSDALE, Ariz. -- On paper, the San Francisco Giants' staff is stacked. It features at least five aces -- veterans who have earned the label based on their past performance. Matt Cain, Tim Hudson and Jake Peavy each rank among the top 20 starters in baseball in WAR over the last 10 years. As good as they've been, none of them have had a single season as stellar as Tim Lincecum’s 2008 and 2009 campaigns, which rank sixth and 13th among the best individual pitching seasons of the last decade. The only other pitchers with more than one top-13 season over that span are Justin Verlander and Clayton Kershaw.

Cain, Hudson, Peavy and Lincecum: That's four guys with established ace-grade credibility on their resumes, and the list doesn't even include Madison Bumgarner. Last year, MadBum delivered his best season yet, worthy of a 4.0 WAR -- which, of course, does not account for what he then did during October to help the Giants win the World Series.

But that postseason performance does provide a window into the Giants' logistical challenge this season. If they are to defend their latest championship, they can't count on Bumgarner to throw 40 percent of their total innings, as he did last October, over six months and 162 games. To succeed during the regular season, a team needs rotational depth -- and San Francisco seemingly has it in that front five, plus its pair of potential swingmen, Yusmeiro Petit and Ryan Vogelsong.

But it may be a very good thing that San Francisco has seven starters to turn to, because each of the front five faces a significant challenge this season as he attempts to retain or regain his effectiveness. Five pitchers, facing five different issues. If they fail to solve all five puzzles, the Giants could find themselves at risk for another odd-year decline.

"The challenge is to get guys ready properly," pitching coach Dave Righetti said. "When you challenge them the way we've done, well, people expect certain things, but the guys can't get caught up in that."

Here's a look at what each pitcher must prove this spring:

Pitcher: Jake Peavy

Challenge: Find a way to retain his effectiveness deeper into games.

Peavy was excellent after coming over from the Red Sox via trade on July 26 -- going 6-4 with a 2.17 ERA and 2.1 WAR in 78 2/3 innings for the Giants -- but throughout 2014 he struggled to get through batting orders a third time, as opponents' OPS went from .662 the first time he faced them to .648 the second time through, before ballooning to .933 the third time he faced them. His strikeout rate tumbled from 20 percent in the first two plate appearances to 15.6 percent the third time.

As part of an effort to improve his stamina and conditioning, Peavy showed up to camp 10 pounds lighter this spring. “I wanted to be light and agile," Peavy said. "I felt like I'd gotten a little heavy the last few years and put a little stress on my ankles.”

Peavy, a three-time All-Star who won the NL Cy Young Award with the Padres in 2007, is still learning as he enters his age-34 season.

"We have a different challenge -- to help him evolve at this point of his career," Righetti said. "He likes to get out there and do his thing, but he’s always open to help. You love to coach guys like that. He's very competitive, but he'll back off and ask what he needs to do to get better."

Pitcher: Matt Cain

Challenge: Become a workhorse once again.

Cain's 2014 season was cut short as he had bone chips -- which had been in his elbow since high school -- removed last August, followed by ankle surgery in September. The 30-year-old also served stints on the disabled list in 2014 for a lacerated finger and a hamstring strain. He looked sharp in his first spring outing, but can the club count on him to return to the form he flashed in leading the team to titles in 2010 and 2012 -- and tossing a perfect game three seasons ago? The Giants will likely need him to log 30-plus starts and 200-plus innings again.

Pitcher: Tim Hudson

Challenge: Fend off Father Time one last time.

Similarly, Hudson, needs to show that his surgically repaired right ankle is sound after having bone spurs removed in January. It's the same ankle that he injured so horrifically in 2013, ending his last season in Atlanta two months early. Hudson pitched through trouble in both hips at different points last year. He's shown plenty of guile in the past, but Hugson will turn 40 in July and his average fastball velocity has been below 90 mph the last three years. Will he follow up on his retirement talk after this season?

Pitcher: Tim Lincecum

Challenge: Get his Freak -- and fastball command -- back on.

From 2008 to 2011, Lincecum was one of the most dominant pitchers of the last decade, going 62-36 with a 2.81 ERA (and a 2.81 FIP). But during that time his fastball velocity dipped from 94 to 92. Then it sank to 90 in 2013 and 2014, and below 90 last year; his ERA rose to 4.76 over the last three years, while his strikeout rate sank from 27 to 22 percent. He’s become even more reliant on breaking and off-speed stuff. While the team hopes that offseason work with his dad to clean up his mechanics will lead to good results, the more basic question is, if his assortment of pitches has fundamentally changed, can Lincecum learn to use what he's got effectively? Or, based on his stellar performance in the 2012 World Series, is the bullpen his ultimate destiny?

Lincecum's challenge echoes that of Dave Stieb, another ace who suffered a mid-career hiccup but regained his dominance by changing his assortment.

"You're going to have these moments where you have to retool a little bit, and refine your game here and there to try and challenge and compete at a high level, and Dave Stieb is a perfect [example] of that," Righetti said. "Dave was my old junior college center fielder, a great athlete ... but with Timmy, everything comes off the fastball, especially against right-handers. With Timmy it's location, because he's got the off-speed stuff, so fastball command is huge for him. This spring is important for him, building toward doing what Dave had to do."

Pitcher: Madison Bumgarner

Challenge: Prove that last year’s 270-inning season won't hurt him.

Will Bumgarner's heroic performance (October-inclusive) at age 24 have a negative long-term impact? The only way to know is to see if the Giants can successfully bring Bumgarner through the 2015 season, with him pitching effectively from beginning to end. San Francisco's pitching coach plans to be hands on with his top gun.

"It's about reading him, and talking to him between starts," Righetti said. "We're being careful with his in-game work. Do you back off on the sides? You have to have them ready, but if we have to back off, we certainly will. We did that in ’13, we stopped him short. He's a 25-year-old pitcher, so it’s important to keep an eye on it.

"[Bumgarner] pitched an extra month, regardless of the innings load. You’re conscious of those innings, but nobody gets to do that unless he's good. That's the dilemma: How to manage a guy on a championship-type club. If he's healthy, how do you tell him that he can't pitch? That's really tough to do in pro baseball. But the arm always comes first, and if he gets to a point where he needs a break, we'll do it. You start to listen to the talk on how to back off, just so that we can watch these guys pitch longer."

There's also Petit, October’s other big hero, who is already looking great in early spring action, and last year's rotation regular, Vogelsong, now a spare starter looking to adapt to the vagaries of a swingman’s role.

All of it adds up to a lot of ifs and maybes and conditional opportunities for success, and that goes a long way toward explaining why the Giants are preparing all seven pitchers as starters this spring. Even a team that doesn't have durability and recovery questions to answer about its rotation has to anticipate the prospect of using a sixth and seventh starter at some point during the season.

"Is it rare that guys stay healthy all year? Yes, but we're definitely built to withstand some injuries," Vogelsong said. "We've got myself ready, and Petit ready, and we'll see what happens.”

So the Giants still have to sort out which five guys they'll open the season with -- which might take all spring.

"Right now Cain and Hudson are about a week behind the other guys," Righetti said. "We'll see if they don't have any setbacks, but if not, we have Petit and Vogey. That'll probably shake out, but we won’t know until the last week.”

The rotation may be settled by then, but the final verdict on how well the Giants handle their quintet of quandaries will only be revealed after six months of regular-season results. Seven may have to do if five aces (of whatever vintage) do not.

Christina Kahrl writes about MLB for ESPN.com. You can follow her on Twitter.