Howe had to take Isak off during the fifth loss.
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He later said, "It was just general tiredness, I don't think there is a problem there. But as I said before the game if he was fit he would play. Fingers crossed he is OK."
Meanwhile, Howe explained Lewis Hall's absence.
He said: "Lewis has a foot problem, ankle problem after the Liverpool game. He did not really recover well after the game so he has been to see a specialist after a scan.
"We are awaiting further news."