Maguire might just do well at the Chelsea defense as he's doing when playing for england.
He may play better in England national team, but I am sure England can play better without him. 
No mate, just don't expect much about him to play well in Chelsea. If there are other options, Chelsea is better to sign other defenders
Players change under the right managers especially defenders. When managers get their pairing wrong it affects players. Did you watch Rudiger play under frank Lampard and Sarri before Sarri left the club and Lampard was sacked (the first time), he was one of the poorest defender I have even seen and wasn't getting playing time because of a flop he was just as Henry Maguire is at Manchester united but then Thomas tuchel came and everything turned around for Rudiger, he was one of the best defender in the premier league at some point and helped Chelsea in lifting the Champions league because our defender were the secret behind our victory that season.
We even sold him to Real Madrid and this was a player we were at some point looking to let go because of how poorly he was performing. I just told you this story for you to understand how vital coaches are to a player. I believe if Henry Maguire meets a coach that can use him to work he'll become a top defender. Let's not forget he wasn't a flop when he was bought so saying he's going to flop if sold is just assumptions. Players change under the right management just as I explained with the Rudiger story. Rudiger has gone on to win another champions league with Real Madrid so don't be surprised if Henry Maguire moves on and become successful as well.
The Chelsea team doesn't require someone to begin regaining his morality and learning effective defensive strategies. The Chelsea team is mostly after quality players for next season. So, for this reason they won't highly go for more quality defender rather than disorganized defenders like Maguire.