This was said by a Welshman

The team plays much better since he's come back.

Shouldn't it be "...since he came back"?

This particular usage does seem quite common among native speakers, doesn't it?
I agree with you: "The team plays much better since he came back."

or there is an error

"The team plays much better since his comeback."

I think "comeback" is a noun.
AnonymousI agree with you: "The team plays much better since he came back." or there is an error "The team plays much better since his comeback." I think "comeback" is a noun.
You could be perfectly right. I hadn't thought of that.
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I looked in my grammar book and 'since' is one of the words after which you can use the present perfect. Or maybe he has not been back long so perfect is good agian.