Gabriel Martinelli was a last-minute withdrawal for Arsenal ahead of Saturday's FA Cup third-round tie against Newcastle United after hurting himself in the warm-up, with manager Mikel Arteta saying the youngster's injury "didn't look good."
Arsenal confirmed that the 19-year-old turned his ankle minutes before kick-off as he prepared for the match.
Martinelli had been in line to start but was quickly removed from the starting line-up, with Reiss Nelson entering in his stead.
Following a 2-0 win for his side in extra-time, manager Arteta provided an update and he was downbeat in his assessment of the problem.
"I am gutted," Arteta said. "I was in my office before the game and one of the coaches came in and said that Gabi hurt himself, that he twisted his ankle.
"I went to the medical room and he was in tears. It didn't look good.
"He was in a lot of pain and we're gonna have to see how he is. He was in pain so I imagine we're not gonna have good news with him."
Asked about when he expected an injury update, Arteta said: "I hope so [by Monday]. We really want to know what's going on.
"He's a character that says he wants to play the next game, that he doesn't care that he can handle pain, but I don't know.
"Hopefully there is nothing too serious but to start with it didn't look too good."
The Brazilian made his first-team return in December against Everton after missing six months of action due to a knee injury, which interrupted his promising start to life at the Emirates Stadium.
He has since gone on to make three starts this season, although he sat out Arsenal's last Premier League clash against West Brom as an unused substitute.
Speaking recently of the injury, which he incurred during a training session in June, Martinelli admitted that he wept upon hearing that it would require surgery and a long lay-off.
"I was about to receive the ball and felt a little pain on my knee. Not enough for me to stop practising, but, while we were on the finishing session, I told them that I was in pain," he explained to the Daily Mail.
"They told me to rest and, if I felt the pain the next day, to let them know.
"When I woke up, I felt it but I was able to walk. I thought it was nothing at first, but it was a pain I'd never felt before. I went to the club for the examinations and they said it was not good and I would need surgery. It broke me. I cried a lot."
Martinelli also paid tribute to manager Mikel Arteta for backing him throughout his absence, while pointing out he cannot afford to rush his return to regular first-team duties.
"Mikel talked to me during my recovery," Martinelli added.
"Always calling me to know how I was, even when I returned to Brazil for a time in August. When the club needed me, he talked to me again, asking if I was feeling ready to play.
"I said, yes. I still feel a bit of pain in the knee, but the doctors say that it is fine. I've not been able to play 90 minutes yet, but it also needs time. My focus is on getting better and helping Arsenal."