Arsene Wenger says Arsenal are still waiting to discover the full extent of Aaron Ramsey's thigh injury.
The Wales midfielder was injured in Tuesday's FA Cup replay win at Hull and, depending on the damage, could miss key fixtures in the Premier League title run-in, plus next week's Champions League last-16 second leg in Barcelona.
"There is a small alert, we don't know how bad it is," Wenger said on arsenal.com.
Defenders Per Mertesacker and Gabriel were also injured at the KC Stadium. The former Germany international had a clash of heads with Nick Powell, while Gabriel was withdrawn following an aerial challenge.
Wenger added: "Mertesacker and Gabriel are very positive, there is nothing wrong there, they are both good."
Laurent Koscielny suffered a calf problem against Manchester United last month and remains a doubt for Sunday's FA Cup quarter-final with Watford.
"He is not far, he will have tests until Sunday, but it could come too soon. He has a little chance," Wenger added.
Goalkeeper Petr Cech (calf), Jack Wilshere (ankle) and fellow midfielder Santi Cazorla (knee) are all nearing returns, but the Hornets clash comes too soon.
"They are all progressing nicely, but this week and next week they have no chance," Wenger said.