Mikel Arteta fears Shkodran Mustafi suffered a nasty ankle injury in Arsenal's 2-1 FA Cup win at Bournemouth.
Mustafi was carried off on a stretcher having landed awkwardly as the Gunners secured a fifth-round clash at Portsmouth.
The 27-year-old Germany defender will have scans on Tuesday to determine the extent of the problem, but manager Arteta conceded the early signs were not positive.
"We'll be assessing him between today and tomorrow but when a central defender has to come off, hopefully I'm wrong, but normally it's not good news," said Arteta.
Asked if Mustafi's situation could alter Arsenal's January transfer plans, Arteta added: "We have a very clear intention of what we wanted to do in the market with the resources we have.
"If we can do it that's fine but his fitness won't change that."
Homegrown duo Bukayo Saka and Eddie Nketiah struck to steer Arsenal past Eddie Howe's struggling Bournemouth.
The Cherries also had a player who came through their academy score as striker Sam Surridge slotted in his first senior goal in added time, but the Gunners held on.
Arteta hailed young English trio Saka, Nketiah and Joe Willock.
"I'm very pleased, I knew it was going to be tough out here," said Arteta.
"We played a really young squad today and wanted to see their reaction, and I'm very pleased.
"In the first half I think we were really, really good. I'm excited that everything we planned came together in a good way.
"They are important, the core of that team at the moment. So to play with that accountability, I really like it.
"The young players were terrific, their work-rate, and for me as well the courage to play, to make big decisions and not just play it safe.
"You look at Nketiah on the wing and then three seconds later he's in the middle of the goal.
"He's always in the middle of the goal to score, and the way he works off the ball is terrific."
Premier League relegation battlers Bournemouth paid the price for a woeful opening half-hour, to leave boss Howe frustrated with their initial attitude.
He said: "The start was hugely disappointing. Maybe the early goal rocked us psychologically but we really struggled to deal with Arsenal in that first half-an-hour. It was all the basics really.
"Once we recovered I thought we were fine, and the better team in the second half, but it was too late.
"I don't think we learned too much tonight because our mentality wasn't right in the first half hour.
"And we'll kick ourselves now because that game was there for us."