Arsene Wenger confirmed Jack Wilshere suffered an ankle injury in Wednesday's match at Chelsea, but is optimistic the midfielder will not miss much time.
Wenger's side held out for a 0-0 draw in Wednesday's first leg but lacked the early control Wilshere gave to their performance when he limped off before the hour.
This setback does not appear to be as serious for the 26-year-old but Wenger told reporters he had suffered a sprain.
"He has an ankle sprain and the first look at it was not too bad, but of course for Sunday I think he will be short," said the Frenchman, who watched the match from the press seats as he continued to serve a touchline ban.
"I do not know how long he will be out. It is a shame to lose him when he is in such good form and it was by blocking the shot that turned his ankle. It's his good ankle so I am quite positive."
Wilshere has entered the final six months of his current deal at Emirates Stadium, and has expressed a desire to commit to fresh terms.
Wenger has promised that future talks will be held during the January transfer window, but the club has been slow to move, reportedly waiting on the midfielder to hit his peak form and fitness.
Wilshere spent last season on loan with Bournemouth and struggled to earn his place back in the Arsenal side early on this season.
However, his return to form and fitness over recent weeks prompted comparisons to the likes of Mesut Ozil and Alexis Sanchez.
"Jack [Wilshere] proved against Chelsea on Wednesday that he is well and truly back, and a force to be reckoned with," former Gunners defender Martin Keown wrote in a column for the Daily Mail earlier this month.
"He is a superstar approaching the levels of Alexis Sanchez and Mesut Ozil but, crucially, he wants to play for Arsenal."