He can only improve under United's coaching. Just look at all the other youngsters. Hernandez, Da Silva twins, Cleverly, Welbeck, Smalling and Evans, all of them within their first year, already bulk up and improve so much in fitness as well as skill. They're not perfect, they still make mistakes but they're getting better.
Jones and De Gea will only get better. It's really very sad when alot of United fans out there dismiss De Gea's chances over a shaky start. Not everyone is a Hernandez lol.