Sevilla midfielder Michael Krohn-Dehli suffered a "very serious" injury in the club's 2-2 draw at Shakhtar Donetsk in the first leg of their Europa League semifinal on Thursday.
After entering the game as a second-half substitute, Krohn-Dehli received the ball in the 70th minute but upon turning fell under his own weight before defenders reached him.
The Denmark international grabbed his left knee while screaming in pain, and he eventually left the pitch on a cart.
Sevilla coach Unai Emery said Krohn-Dehli appears set for an extended period out of action.
"Krohn-Dehli's injury looked very serious," he added. "We have to be there for him now and we want to progress for him. It will serve as a motivation to us."
Sevilla had fallen behind 2-1 before Kevin Gameiro converted a late penalty to salvage the draw and give the Spanish side an away-goals advantage.
Emery said he expects the return leg to be another close contest.
"Both teams showed that they deserve to be in the semifinals," Emery said. "The tie is still very open, it's 50-50. We will have to be at our very best in the return if we are to progress.
"I was not surprised by the first half, nor by the second half. I expect a similar game in the return."
Vitolo, who scored Sevilla's first goal and drew the late penalty, said his side did not lose confidence after falling behind.
"Shakhtar have some great players and we suffered," he told AS. "We never gave up, though, and the late goal was a great reward that puts us in a good position.
"The coach always tells us to stay calm and wait for our moment and he got it right."
Shakhtar manager Mircea Lucescu said his side were punished for a late mistake and conceded that a draw was a fair result on the night.
"I think the final score reflects the game," Lucescu said. "We created a lot of chances in the first half and could have scored four goals rather than just the two we got.
"But in the closing stages of the match we tired a bit and made a lot of mistakes in defence.
"We still have every chance of making it to the final following this result, though. We have been doing well in away games.
"The players are upset because of the late goal, but we have to get over this and move on. We still have a good chance of making it to the final."