Obviously I'm delighted that Darmian has settled in well and he was certainly a bargain compared to all the other RBs we were linked with but even still I can't help but wonder how much better we'd look with a serious attacking threat from RB.
Darmian is perfectly capable of being a serious attacking threat. You saw how he can turn it on at will when he waltzed straight past two defenders to cross against Sunderland with appearing to break a sweat. He's very two-footed, unusually for a fullback, and it makes the attacking that much easier. For Italy, it's really his attacking football that has won him the plaudits.
In our initial run of games, it looked pretty undeniable to me that both he and Shaw had been instructed to play conservatively. They noticeably chose not to go forward a number of times, both with the ball or looking for it. Shaw's game got more attacking more recently, perhaps as a result of it becoming clear that this season he's an unstoppable beast, and Van Gaal responding to that. But anyone who watched Darmian much before he came to us will be able to see that he's consciously focussing on his defending.
Which isn't a strategy you can really criticise - if he gets caught forward on the right, he's got one of the most defensively negligible 'wingers' in the league as his insurance. Basically, the opposition would be left with an unguarded flank to attack. Compare with, say, Shaw-Young on the left; Shaw could be reasonably confident that if he lost the ball in an advanced position, Young would bust a gut to get back and cover, and have the defensive ability to fill the gap.
This is why I'm always split on Mata. Even after good performances like against Sunderland, you can't deny that however good he is at the things he's good at, he will always be really bad at the things he's bad at, and other players have to take the slack. If you play someone else there, Darmian probably has a lot more licence to attack, and he's capable of being a team's most effective outlet (he often is for Italy these days, especially when he plays with Candreva). So you have to balance whether the loss of Mata is worth that gain.
For the record, I'm not saying we should drop him. You should never drop someone who's hitting hot form, even if in the long run you don't think they're the answer. But I'd want us to be willing to drop him quickly if he stops making such a big statistical contribution in attack.