Arsenal's 2-0 victory against Chelsea was marred by an injury to Hector Bellerin, which saw the Spain international stretchered off.
Bellerin, who was making his first start since suffering a calf injury last month, went down in the second half after overstretching as he tried to dispossess Chelsea's Marcos Alonso.
The full-back clutched his left knee and lay in agony on the Emirates pitch before receiving treatment for several minutes from Arsenal's medical staff.
Bellerin received a standing ovation as he was was stretchered off with 20 minutes left to play, with Mohamed Elneny coming on to replace him.
Speaking after the game, Arsenal boss Unai Emery gave a downbeat assessment of the injury sustained by the Arsenal right-back.
"It is his knee and the first prediction is it is an important [bad] injury," Emery said.
"We have to wait until tomorrow, but first impressions are not positive."
Arsenal's victory saw them reclaim fifth place from Manchester United and close the gap on London rivals Chelsea to three points in the race to finish in the top four.
Emery, who now wants consistency in performances, was full of praise for his players' efforts.
"Today, every circumstance came with us," he said. "The mentality was good.
"And we want to have the same mentality today at home and away - which we haven't yet."
The Gunners are in FA Cup action against Manchester United on Friday, before they host Cardiff in the Premier League four days later.