Graeme Souness says Manchester United will need to spend at least two seasons rebuilding their squad before becoming title contenders again.
Netherlands coach Louis van Gaal looks set to be named as David Moyes' replacement after a disappointing season which saw the champions drop to seventh place in the table, their worst finish of the Premier League era.
However, Souness believes the club must learn from the mistakes of last summer when they failed to sign their top targets.
And he says supporters may need to accept that it could take four transfer windows before they have brought in enough quality players to challenge for the championship again.
"It's not a quick fix," he said on the Sky Sports End of Season Special.
"From where Man United were to where they are now, the strength is nowhere near where they would want it to be.
"You put Man United in that elite group of Barcelona, Real Madrid and Bayern Munich, but how many Man United players in the current squad would be wanted by any of those teams? I can think of two - and beyond that you're struggling.
"I don't see it as a quick fix. I think you have to look at it in transfer windows and I think it's a minimum of four transfer windows, if it goes well, before Man United get back to where they want to be and should be.
"Man United will still be a pull for a number of years, even if they aren't in the Champions League. They still have the finances to pay the big wages.
"The biggest problem they will have is will the quality be available? Then you're faced with the dilemma as a manager and a football club as to whether to sit and do nothing because you can't get the one you want, or go for the third or fourth or fifth choice. What do you do? That's a difficult call.
"The supporters will want signings, that's where the pressure comes from. That's why they ended up taking [Marouane] Fellaini. The supporters are impatient."
Former Manchester United captain Gary Neville agrees that the new manager must be patient when he enters the transfer market this summer.
He admits the season has been a disappointment following the departure of Sir Alex Ferguson, but would prefer to see quality additions rather than wholesale changes.
Neville said: "I do think Manchester United have a really good chance of getting back in the Champions League next season, I really do believe that. I think not having European football will help them.
"If the new manager feels they need eight players, that doesn't mean they should go and sign eight players. Quality is the right thing, not the quantity.
"We've seen many clubs sign lots of players and bedding in is always a difficulty, so sign three or four real top-quality players and improve next season and then maybe go again with two or three the year after.
"That's the more organic approach that I would expect Manchester United to have.
"However, I wouldn't have expected Manchester United to sack their manager after 10 months, so what's going to happen in the next few months... who knows?
"I'm not in a rush. As a fan I'd rather see James Wilson come through than sign some fourth-choice striker that isn't at the top of the list."