The 25-year-old was forced to pull out of the 1-0 defeat to Arsenal after undergoing a scan on a groin injury on Sunday morning.
Howe refused to answer whether Isak will be ready to face Everton in their Final game of the season with Champions League qualification still in the balance.
Speaking after the game, Howe said: "That's unknown. I don't at this moment in time.
"He only felt pain after training, he'd felt fine during training and it was just a bit of stiffness really. So he went for a scan this morning purely a precaution. Then it became obvious that he was not going to be fit to play. I don't know.
"A lot depends on how he will react in the next few days. He has had a couple of groin injuries either side, this is one he had earlier in the season. "