Mikel Arteta expects to be without midfielder Thomas Partey for Arsenal's "next few matches" after the midfielder aggravated a groin injury last weekend.
Partey missed four games before returning to Arsenal's starting line-up ahead of schedule for Sunday's 2-0 north London derby defeat to Tottenham.
He was forced off just before half-time and is now facing another brief spell on the sidelines, but Arteta has defended his decision to start the Ghana international against Spurs.
"His injury was the same area as before but not the same spot. He'll miss the next few matches," Arteta said ahead of Thursday's Europa League clash with Dundalk.
"He did incredibly well and he was very confident. We tested him three times and he had zero symptoms, but in football you have a lot of unpredictable actions.
"He went into the floor, his knee got stuck, afterwards he stretched and got up. Immediately he put a lot of stress in a similar area and it happened.
"Afterwards people will say it was a bit early but I don't think it was because he was completely fine to play."
Arteta accused Partey of failing to grasp the severity of the situation after leaving the field in the build-up to Tottenham's crucial second goal.
That led to talk of discontent behind the scenes between the manager and his big-money signing, but Partey took to Instagram on Wednesday to praise Arteta's work.
"Being part of the Arsenal family is something incredible for me," he posted. "Since the day I arrived I have felt so supported.
"The coach had a great influence in my decision to join this team and even though we had some bad results I know we are building something good together.
"I have had the bad luck to suffer an injury but I hope I can play again soon and fight for this team."
Pierre-Emerick Aubameyang fired another blank in the defeat to Tottenham, making it four games without a goal for a low-scoring Arsenal.
He has just two goals from 20 shots in the Premier League this term, nine of those finding the target, but Arteta is confident his skipper will soon regain his best form.
"Nobody is un-droppable. At the end of the day, we have to find the right players to win football matches," he said.
"But we cannot forget what Auba has done for this club and what he has done in recent months, not years ago.
"I see how he's training and how he's behaving, how much he wants to turn this situation around.
"When I see that type of hunger, it's about supporting the player as well as the status he has in the team because of the performances he has provided for this football club."
Arsenal have already made certain of top spot in Group B ahead of their match with Dundalk in Dublin, boasting a perfect record after five matches.
Another victory at the Aviva Stadium would make it six consecutive wins in European competition, excluding qualifiers, for the first time since November 2005.
Arteta is expected to heavily rotate his side for the final group outing, with Dundalk bottom and yet to pick up a point.
"We have a few kids here," the Spaniard said when asked about his team selection. "We left a few players at home as well.
"There are a lot of games and we need a complete training week as well, which I think is going to benefit us. But we're going to have a very competitive team to play."
The Gunners' European form is in stark contrast to their domestic woes, their return of 13 points from 11 league games their fewest since 1981-82 (12 points).
"The demands for this football club are the highest," he said. "We are not getting the results in the league that we all expect, of course the pressure is increasing.
"But more than the pressure, I would say the responsibility of all of us to make it happen next Sunday and start winning football matches, which is the most important thing."