Arsenal boss Unai Emery says Dani Ceballos has what looks like a "small" hamstring injury sustained in the Europa League draw with Vitoria SC.
The midfielder initially pulled up early in the second half of the 1-1 result in Guimaraes and was replaced by Matteo Guendouzi 54 minutes in.
The Real Madrid loanee appears likely to miss at least Saturday's Premier League trip to Leicester City, although Emery says he will have further tests on Thursday.
"We are going to wait for the analysis tomorrow but, at the beginning, I think it’s a small injury," Emery told a news conference.
Arsenal had looked set to claim a win that would have guaranteed their place in the next round when Shkodran Mustafi headed in Nicolas Pepe's free-kick in the 80th minute.
However, Bruno Duarte equalised in injury time and the Gunners came close to a defeat in the closing moments, with Mustafi deflecting a cross onto the post and Rochinha sending a volley narrowly wide.
Emery was happy with his players' efforts in torrential rain, even though Mustafi's header was Arsenal's only shot on target.
"The players worked and tried and we were very close to winning. In the last minute, they pushed a lot and they scored," he said to BT Sport.
"The objective is to be first in the group and we are. At the end of the match, we're thinking about Saturday and Leicester.
"We tried [to score] but also they are a very competitive team. We did better defensively. Offensively, this team are very organised and it's not easy to have a lot of chances. It's not good because we want to win but we are first, and this is our target."
Mustafi felt the Gunners could have defended better to prevent Vitoria equalising but the centre-back is content to have taken a point that moves them closer to reaching the next round.
"I would love to score in the 90th minute. When you score a goal and you still have to play against a team like today, they threw everything up front, sometimes it's not organised any more and they throw a lot of bodies into the box and they keep crossing," he said.
"I think we could have dealt with [the danger] a little bit earlier, instead of letting them cross again. In the end, you have to accept it and move forward and take the positive things.
"You have to say they are a good side, they're in the Europa League for a reason, they're quite compact. We played today with a back three and we needed some time to get the rhythm in. In the first half, we didn't do it as we wanted; the second half was better, better rhythm, better passing.
"We just need to stay calm. Everyone wants to show and give everything and sometimes, when you're over-motivated, it's difficult to deal with that. At the end of the day, we need to stay calm, play our football, believe in ourselves and then in the final third try and do it a little more intelligently, with a little more accuracy in our final pass."