Jose Mourinho says he has restored "happiness, trust, belief and resilience" to a Manchester United squad lacking it under Louis van Gaal.
The United boss said that new players arriving in the summer will join a squad "ready to go for big things".
Van Gaal was sacked last May after winning the FA Cup but failing to qualify for the Champions League.
In his first season at Old Trafford, Mourinho has won the League Cup and could still finish in the top four.
He has also guided his side into the Europa League semi-finals, where they will face Celta Vigo.
Mourinho said: "I think Mr Van Gaal left a good group of boys with very good relations between them.
"[But] I think they missed happiness, they missed trust, belief, this extra bit that brings resilience, brings fight and they have it [now].
"When we meet again (for pre-season) on 9 July they will be a stronger group. The new players, when they arrive, will find a group more ready to go for big things."
United drew 0-0 with Manchester City on Thursday to extend their unbeaten run in the Premier League to 24 games
They are fifth, one point behind City and two points behind third-placed Liverpool with a game in hand. They face Swansea on Sunday.