The midfielder's first season with United has been plagued by injury.
"He was struggling from the second game of the season," Ten Hag said. "He returns for a period and then picked up another injury, so of course that's not great.
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"That is very frustrating for him and I really feel for him, so I really want to be there for him because he had such high expectations - and I had high expectations.
"I'm sure he (will be) a very good player for us. But first of all - and we have to do it very securely - is to get him fit.
"Secure in the process - step by step, don't force it - and then hopefully, by the end of the season, he can stay fit and find himself in the team - or at least he plays a role in this team."