United have already done most of the groundwork on their first big deal of the summer – a £25m move for PSV winger Memphis Depay, who completed his medical at Carrington earlier this week.
And speaking at the club’s end-of-season awards ceremony, the manager promised further changes as he attempts to deliver the Premier League title in his second year.
Van Gaal told MUTV: “When you purchase other players, you get another stimulus in the group, and you need that.
“Our selection is out of balance and we have managed to be fourth in the league, but we have to manage to be the champions… so it shall be a rough summer, I think, for a lot of players but I think a top club like Manchester United have to do that.”
The United manager did add that he had been impressed with the “fighting spirit”, “the attitude and the motivation” of his players and singled out goalkeeper David De Gea for special praise.
He said: “In the beginning of the season, as a manager I observed him, I analysed him and then I saw his progress in the season. I have to say it is a fantastic development from him. As a manager, you look at it from that perspective and that is why David De Gea has impressed me the most.”
But the Dutchman has already warned it may prove difficult to prevent De Gea joining Real Madrid this summer, and he again offered no guarantees about the keeper’s future.
Referring to the upcoming transfer business in general, he said: “I can only promise that I will do my utmost best, that we shall fight to the end – more we cannot do – and that we make the right decisions when you purchase players.
“And when players are leaving the club, it’s a natural process, the fans are not forgetting that.”
De Gea was voted Player of the year by his team-mates and the club’s fans, and he said: “I want to say thank you to my team-mates, they have been fantastic this season, we have a great dressing room.”