Honestly, I think he has had almost zero impact on the team. Our defence saw minimal improvement at the start of the season, conceding in 17 of 19 games. We were on course to concede 46 goals.
From the 14th Sep to 27th Dec we conceded in every game. It wasn't until Fernandes and Matic were in the team where the goals conceded dropped. I think Maguire's impact has been minimal, and we would have seen defensive performances no worse with Smalling.
I think it has been a pretty pointless signing. We should have waited until someone truly outstanding came along.
I think this will go down as a poor signing in the long run.