Jose Mourinho says Manchester United stood still and failed to evolve under both Louis van Gaal and David Moyes.
United have struggled to replicate the success that saw them become Premier League giants under Six Alex Ferguson, who guided the club to the last of their 20 top-flight titles in 2013.
Ferguson's successor Moyes was sacked after 10 months in charge while early promise under Van Gaal fizzled out, even though the Dutchman ended his reign by winning the FA Cup.
On Sunday morning, Van Gaal hit out at United over his sacking, accusing the club of putting his "head in a noose" before he was "publicly placed on the gallows."
Mourinho, who won the EFL Cup and the Europa League in his first season at Old Trafford, believes he inherited a side that were burdened by past successes.
He told the Sunday Times: "There was an evolution in the other clubs; there was no evolution in this club. In all the areas that make a team successful, I think we stopped in time.
"That was quite an empty period in the club with no evolution in areas that are important. It was the weight of the past on the shoulders of the players, managers, even boards, even probably owners, because when you are used to 'win and win and win', and suddenly you stop, it's like a heavy burden.
"Today, we have better working conditions, a better medical department, a better analysis department, a better scouting department, a better media department, and in many cases we did it without changing the people, which is quite important."
Former Chelsea boss Mourinho has added Romelu Lukaku and Nemanja Matic, two players he worked with at Stamford Bridge, to his squad this summer.
"Lukaku was my player as a kid," said the former Real Madrid boss.
"He left [Chelsea] because he was thinking he deserves more opportunity, but he's such a good guy that during these years we were in close contact with a good relationship. So I knew he was an intelligent guy with a positive attitude."
On Matic, Mourinho added: "I cannot say that I had better guys than him in my career."