I can see what you're saying, but I still totally disagree. Any slight advantage Valencia's better pace gives him is massively outweighed by him just being a much worse defender.
So far Montero has come up against Van Aanholt, Janmaat and an all-over-the-place Ivanovic. Darmian has been significantly better than any of them this season - in fact, there's really not a right-back in the league who's been better, defensively. In theory, possibly a faster player (and let's not forget that Darmian is no slouch, there's not a massive margin between them) would be a good shout against Montero, but not when that player is so much worse on all other counts.
If you really want to make changes to deal with Montero, you should be looking at Mata, not Darmian. The most impressive thing about Darmian's start to the season is that he's been so solid with one of the least defensively useful wingers in the league in front of him. Personally I think that, as ever, we should be looking to play our own game against a team who, while in dangerous form, are not our equals. But if you wanted to make adjustments, the obvious one would be to bring in Young for Mata for the sake of his excellent defensive contribution.