Newcastle boss Eddie Howe concedes Anthony Gordon's knee injury "doesn't look good".
The winger broke down during last night's 3-2 defeat at Chelsea.
Howe said on Gordon: "He is in a bit of pain and we fear it is not looking too good but it is very early days.
"That was a huge moment in the game to lose him then. He has been so good for us this season and so consistent. That is another psychological blow for the players."
Howe added: "I think it (Gordon's knee pain) was quite early on in the game. But as I said, there was no moment where he felt an injury, just the pain was there. So we sent someone over because he didn't look like he was moving right. And then eventually he succumbed to the injury."
"He's in a bit of pain but it's not looking too good. It's early moments, we'll wait and see. It was a huge moment to lose him, given what he has done this season. It was a psychological blow."