It seems to me that even if Man U remain unchanged next season, their game shouldn't get any worse... As far as I know, the main players are not leaving the club yet... Man U showed good football last season and finished the season in third place... If they don't have any major team changes, they can continue to play well without any new transfers...
Their problem is similar like Arsenal where they don't have a good player in bench, if there's a player in Manchester United is injured, their performance would be bad. However Manchester United make a crucial move where they let De Gea to leave, goalkeeper is one of the most important position similar like a central midfielder. I don't think Manchester United's performance would be same, it's either they're better or worse.