New Manchester City signing Nico Gonzalez was taken off injured just 22 minutes into his debut on Saturday.
The midfielder, who joined from Porto in a €60 million ($62m) deal on transfer deadline day, appeared to hurt his ribs after falling awkwardly in a challenge with Leyton Orient's Sonny Perkins.
After receiving treatment, Gonzalez held his side as he left the field early in the FA Cup fourth-round match at Brisbane Road.
"I think Nico got immediately what is the Premier League [and English football]," City manager Pep Guardiola said. "I don't know how hard [the knock] is, but he could not continue."
His injury will be a concern for Guardiola ahead of Tuesday's Champions League playoff against Real Madrid.
The signing of Gonzalez was intended to help fill the void left by Ballon d'Or winner Rodri, who has been out since September because of an ACL injury. In Rodri's absence, City has seen its Premier League title defense unravel and needs to beat holder Madrid in a two-legged playoff to advance to the round of 16 in the Champions League.
Gonzalez was one of five signings in the January window as Guardiola attempted to salvage the season.
City spent more than $200 million in a spree that also saw the arrivals of Egypt forward Omar Marmoush for $73 million and defenders Abdukodir Khusanov from Lens and Vitor Reis for a combined $77 million. Christian McFarlane, an 18-year-old left back, joined from sister club New York City FC while captain Kyle Walker left for AC Milan.
City fought back from going a goal down in the first half to win 2-1 and advance to the fifth round.
