Realistically we won't be able to replace his creativity, at least not immediately. The hope would be that the loss of creativity might be ameliorated somewhat by also losing the flaws inherent in his often "languid" style. I don't have any doubt we'll be immediately worse off with him gone though.
However, he wants to leave so he will be leaving, whether it's this summer or next. There's nothing the club can realistically do about that so they'll have to bite the bullet at some point. Personally I'd be inclined to get it over with this summer, take whatever hit it brings and start moving on asap. Though it really depends on the offers we're actually getting, the offers we think we might get if we play hardball and how committed we think Pogba will be if he doesn't go this summer.
It's a tough situation for the club and they will be criticised for whatever they do.