Mainoo was taken off while limping, with substitute Mount suffering a head injury late on.
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Ten Hag, on Mainoo, later said: "I can't say in this moment.
"I have to find out but, of course, when a player is coming off before half-time already, then there are some concerns.
"Also, I have seen how Mason came off. He was bleeding so it's clear there are some problems there and we have to see how they recover in the coming days."