The USA midfielder broke down during last night's 4-4 draw at Manchester United, where he was linked to on the eve of Monday's clash.
After the match, Iraola told the Daily Echo: “I think it's the worst news from the game, definitely.
“I think he has a knee twist. I think it's just the MCL. But we have to check him and see if it's just a small sprain or something bigger.
“It's just a clear play. Obviously it's not what you want in, I think it was the second or third minute, to lose a player like Tyler for us, it’s massive.
“Especially when you don't have Lewis Cook, he’s suspended. So Alex Scott again, like against Chelsea has to play in that holding midfielder position.
“And I think he has done it really well. But it's bad news for us, because Tyler is massive for us.”