He's fitter indeed, but I think that the solid performances have more to do with his position on the pitch. His passing % is even worse than it used to be, but we don't even feel it now, because these are not short passes in dangerous positions.
Indeed, he won't score as much now that he plays on the flank. However, he'll have much more space to show some of his other qualities.
He's in top 5 in quite a lot of categories (per 90 minutes; including only those who played more than 120 minutes). When he played in midfield he was among the worst in many of the below categories.
Tackles:
1. Rabiot 3.3
2. McKennie 3.0
3. Cambiaso 2.9
Interceptions:
1. Alex Sandro 2.5
2. Danilo 1.8
3. McKennie 1.3
Clearances:
1. Bremer 3.5
2. Danilo 3.0
3. McKennie 2.6
Blocked passes:
1. Kostic 1.4
2. McKennie 1.3
3. Rabiot 1.3
Blocked crosses:
1. Miretti 0.5
2. Rabiot 0.5
3. Gatti 0.5
4. Cambiaso 0.5
5. McKennie 0.4
Unsuccessful touches (players with least such touches)
1. Kostic 0
2. Danilo 0.3
3. Locatelli 0.3
4. Bremer 0.5
5. McKennie 0.9
Dispossessed:
1. McKennie 0
Won aerials:
1. McKennie 3
2. Danilo 2
3. Bremer 1.5
Created chances:
1. McKennie 2.2
2. Kostic 2.1
3. Rabiot 1.8