Louis van Gaal said Rio Ferdinand left Manchester United at the right time and does not wish the centre-back was still at Old Trafford.
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The 35-year-old ended a 12-year stay with United by going when his contract expired in the summer and he returns to his former club for the first time on Sunday in the colours of QPR.
United have been short of centre-backs this season, when Jonny Evans and Chris Smalling have suffered injuries and new signing Marcos Rojo was ineligible before receiving his work permit but manager Van Gaal said he does not regret Ferdinand's departure.
"No," he said in a news conference. "There is a time to stay and a time to go and I think he has decided to go. He was a great player and maybe he is still a great player but there is always a time to go with a club."
Van Gaal is yet to win as United manager but, although his side have only taken two points from a possible nine this season, the club have budgeted to finish third.
But while the Dutchman believes it will take time for the players to adapt to his methods, he said he has no problems with vice-chairman Ed Woodward's expectations.
He added: "We have spoken about the target when we have the meetings together and I think a club as Man United has to put targets. I also put targets so I don't think it is an unreasonable target.
#INSERT type:image caption:Louis van Gaal said Rio Ferdinand left Manchester United at the right time and does not wish the centre-back was still at Old Trafford. END#
"It shall be difficult, that is another question but I think a club as Manchester United has to put a target. Within the [top] three is logical because Manchester United wants to be in the Champions League."
Van Gaal could give four of his summer signings -- Radamel Falcao, Daley Blind, Luke Shaw and Rojo -- debuts against QPR on Sunday but was keen to stress that United have also trimmed their squad and that a new-look team may not gel immediately.
He added: "I know as a coach when you let 14 players go and buy six players then you have 20 different relationships in the dressing room. The hierarchy in the dressing room is gone and now we have to build up a new hierarchy. That needs time."