Crystal Palace manager Roy Hodgson has confirmed Michael Oliseh has suffered a “serious” hamstring injury, which will be “devastating” for everyone involved.
Palace were trailing 3-0 when 22-year-old winger Oliseh was brought on, having also been sent off in the previous game.
“So many things that should have gone wrong went wrong,” Hodgson said.
“But we don't have hindsight, and that's the problem.”
After the match, Hodgson defended the decision to bring on Olise, saying “everyone was very happy'' with the London-born former France youth international being brought in as a substitute.
“They are [the medical team] I told him he was fine with being on the bench and playing in the second half. Unfortunately, I don't have a crystal ball,'' Mr Hodgson said.
Hodgson spoke again on Thursday, denying claims by supporters that Oliseh was already suffering from a physical problem ahead of Monday's game against Chelsea, but admitted he now has a hamstring injury.
“It's really devastating for him, for the club and for everyone,” Hodgson said.
“I don't think he had an injury, I think it was more about fatigue.
“When you have a player like Olise on the bench, it's not easy to turn around and say, 'Just sit there.'
Oliseh has only played in 11 games this season due to injury, but he has six goals and three assists.
“Michael is a quiet guy and that's the other part of this situation,” Hodgson said.
“Another player might have come and said, 'Look, I don't think it's worth it. I need more time.'
“But it's unfair to him. He's just following what the club thinks.”
The south London club also lost winger Ebelesi Eze to a muscular injury in the win over Sheffield United, while defender Marc Ghehi picked up a knee problem against Brighton.
Hodgson is under pressure after nine defeats in 15 Premier League games, leaving Palace in 14th place, five points above the relegation zone.
Fans have expressed their displeasure in recent weeks, holding up banners during matches between Arsenal and Brighton protesting the way the club is being run.
The former England manager also told Hodgson when he substituted Eze in the 64th minute after the FA Cup third-round replay defeat to Everton last month: “I don't know what I'm doing.'' No,” he yelled.
Mr Hodgson said the speculation was “outside my control” and he had no intention of stopping his work.
“My guess is that there will always be people who don't do well at the club and then there will be speculation about the future of those people, the manager, so I think I need to do something different. Why?” ” He said.
“It's not within my control, I mean, it doesn't affect my day-to-day job. Even if I come here, I think we need to work with the players, so it's not something I'm going to do with the players. I won't stop working, it won't stop me.'' I don't just choose the team, I choose the tactics and try to get my team into the game in the best possible shape.
“These are external things and you have no choice but to overlook and accept them as a fact of football life.”