Jose Mourinho believes Manchester United cannot return to their former greatness due to the size of the rebuilding job required at the club and the increased level of competition in the Premier League.
The United boss feels he took over "a sad club" when he arrived at Old Trafford this season and thinks there is still significant work to do.
Mourinho has already won the Community Shield and the EFL Cup since being appointed as Louis van Gaal's replacement, but his side are sixth in the league and 17 points behind leaders Chelsea, his former employers.
And he says it will be impossible for his side to enjoy the same level of unprecedented success as they did with Alex Ferguson at the helm, the Scot winning 13 Premier League titles during his 26 years in charge.
"Forget it," Mourinho told BBC Sport when asked if a return to those glory days could be achieved.
"Don't try to go 10, 20 years ago because it is not possible any more.
"We are not ready to be Manchester United. We are not ready to be a dominant force. We are not ready to try and win everything."
Speaking before Thursday's Europa League win over Rostov, Mourinho added: "I found a sad club. Manchester United sold players that I would never sell, bought players that I would never buy.
"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.
"Many other teams in England are going to finish the season without a trophy. But we have to fight for the top four, we have to fight for the Champions League. The EFL Cup is not enough to say that the season is over."