Manchester United "are not ready to be a dominant force" and fans should forget about a return to the days of Sir Alex Ferguson, manager Jose Mourinho has told BBC Sport.
United won 13 Premier League titles under Ferguson, but Mourinho says it is now impossible to be so dominant.
Asked if he could return the club to its former greatness, the Portuguese said: "Forget it.
"Don't try to go 10, 20 years ago because it is not possible any more."
In a wide-ranging interview with Gary Lineker for the Premier League Show, Mourinho also said:
- United do not need to qualify for the Champions League to attract top players.
- He would not have sold forwards Angel di Maria, Javier Hernandez and Danny Welbeck had he been United manager.
- It has not been easy for midfielder Paul Pogba to adjust back to English football.
'We are not ready to be Manchester United'
Mourinho, 54, signed a three-year contract last May to replace Louis van Gaal, who was sacked despite winning the FA Cup.
The Red Devils have finished seventh, fourth and fifth in the three full seasons since Ferguson's retirement, and have been in sixth place since 6 November.
Mourinho does not believe a return to winning the Premier League every year is close, but does not want the season to peter out after winning the EFL Cup last month.
"We are not ready to be Manchester United," he said.
"We are not ready to be a dominant force. We are not ready to try and win everything.
"Because of the nature of the club, and of myself, we are ready to fight for every game, every point. But there is a space between the general ambition of such a giant club and what we are in reality."
Mourinho said United - who beat Premier League champions Leicester City in the Community Shield in August - had won "one and a half" trophies this season.
"Many other teams in England are going to finish the season without a trophy," he said. "But we have to fight for the top four, we have to fight for the Champions League. The cup is not enough to say that the season is over."
'I found a sad club'
Since Mourinho took charge, United have spent an estimated £150m on midfielders Paul Pogba and Henrikh Mkhitaryan and defender Eric Bailly, and brought in striker Zlatan Ibrahimovic for free.
But the Portuguese said the club's previous transfer dealings caused him concern.
He named three forwards - PSG's Angel di Maria, Bayer Leverkusen's Javier Hernandez and Arsenal's Danny Welbeck - as players he would not have sold.
"I found a sad club," he said. "Manchester United sold players that I would never sell, bought players that I would never buy."
Mourinho would not name the players he would not have signed, but in January he allowed midfielder Morgan Schneiderlin and forward Memphis Depay to leave for Everton and Lyon respectively.
He has largely frozen out former Germany captain Bastian Schweinsteiger and has handed just seven league starts to left-back Luke Shaw, who cost £27m from Southampton in 2014.
Mourinho said he was not worried about attracting players if United fail to qualify for the Champions League, pointing to last summer as proof the club can still sign the best players.
"Manchester United is very powerful, it doesn't need to be in the Champions League to attract the best players," he said.
"Zlatan could still be in Paris. Mkhitaryan could be at Borussia Dortmund. Pogba could be at Juventus. We were able to attract the players because they know that Manchester United sooner or later will get there.
"If any player decides not to come because of that, then I am happy that they are not coming."
'Pogba doesn't disappoint me at all'
Pogba, 24, has scored seven goals since joining United for a world-record £89m last summer, but has been criticised for a perceived lack of impact in matches.
Mourinho says the France international will improve.
"It isn't easy for Pogba," he said.
"The country is so different to Italian football. It is hard for him. I'm not disappointed at all. The most important thing is his personality. He is professional and he will improve for sure."
Mourinho also praised the contribution of 35-year-old Ibrahimovic, who has scored 26 goals this season.
"Zlatan is not a surprise for me," he said. "I know the personality, I know the body, I know the ambition that brought him here.
"Could he do it in the best league in the world? He has done it everywhere else. He's doing amazingly well."