Mourinho, who has already won the League Cup in his first season in charge, believes the club is still far from ready to compete for major trophies on a consistent basis.
"We are not ready to be a dominant force. We are not ready to try and win everything," the Portuguese, the club's third permanent manager since Ferguson stood down, told the BBC.
"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, who won three league titles in two spells with Chelsea, also questioned the club's transfer business under his predecessor Louis van Gaal, who sold strikers Danny Welbeck and Javier Hernandez as well as winger Angel Di Maria.
"I would never have sold Di Maria, Chicharito (Hernandez), Danny Welbeck -- never, no chance," Mourinho added.
"I found a sad club. Manchester United sold players that I would never sell, bought players that I would never buy."
Mourinho signed Zlatan Ibrahimovic, Paul Pogba, Henrikh Mkhitaryan and Eric Bailly at the start of the season and allowed midfielders Memphis Depay and Morgan Schneiderlin to leave the club in January.
United beat Russian side Rostov 1-0 on Thursday to reach the last-eight of the Europa League and are sixth in the Premier League with 12 games to play.
(Reporting by Hardik Vyas in Bengaluru, editing by Nick Mulvenney)
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