José Mourinho has identified Tottenham as strong rivals to win the Premier League title next season and compared his challenge at Manchester United to the one he faced previously at Real Madrid, insisting the club had been left behind in “the evolution of football” when he succeeded Louis van Gaal last summer.

Mourinho has previously won the championship in every second season of his managerial career but believes this will be difficult in the forthcoming campaign. The manager feels United are candidates to win a 21st league title but identified Spurs as major contenders because they have retained all their players so far apart from Kyle Walker, who Mourinho asserted was sold to Manchester City because his replacement, Kieran Trippier, may be as good.

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Asked how he assessed his chances of making United champions again, Mourinho said: “I don’t think we are big candidates for the title but we are [candidates]. Everybody speaks about the dimension of the investment at Man City [so far just below £220m] but there is another team that I feel the dimension of their investment is also phenomenal – Tottenham. Until now they spent zero pounds, right? They keep everybody they want to keep. They keep Dele Alli, [Harry] Kane and [Toby] Alderweireld, they keep Eric Dier, they keep everyone they want to keep.

“They sold Kyle Walker I think because they wanted to sell. And probably because they think Trippier is as good as Walker. And he’s younger than Walker. Chelsea are champions and they buy now. We are halfway through the market and they buy [Antonio] Rüdiger, [Tiemoué] Bakayoko and [Álvaro] Morata. So every club makes different kinds of investments but all of them are investing a lot to win the title.”

Mourinho joined Real in the summer of 2010 and claimed the title in his second season, gathering 100 points, then a record for any of Europe’s top leagues. He admitted the challenge at Old Trafford has been just as difficult having inherited a side from Van Gaal that missed out on Champions League qualification.

“The hardest part in my opinion was the period where there was an evolution in football overall, in terms of every other team, not just the typical powerful teams but all the others. Their evolution in relation to the changes in football allowed them also to be economically strong and to develop,” he said.

“At the same time there was an evolution and Man United did not follow that evolution and my work is not just about arriving at a football team and developing the football team to attack, football targets and objectives; it was more than that. It was to go into the structure and to try to adapt the structure around the football team to a new dynamic.

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“That was the same at Real Madrid,” Mourinho added. “A big club with big expectations but a difficult period with the evolution of other teams and the distance between Real Madrid and Barcelona growing bigger and bigger and bigger. When I arrived it was not just about developing a football team to try to stop that domination but also to develop everything around the team and in that respect I can see similarities in terms of the difficulties and similarities in terms of the dimension of my job.”

Last season United claimed the EFL Cup and Europa League and Mourinho warned his team will be stronger in 2017-18. “We are better than last season and the spirit is fantastic,” he said. “The confidence levels are good and the fact we had some kind of success last season also was important to relax some personalities. They feel more confident to go into the Champions League without being a candidate. That’s good.

“We went into the Europa League with the tag of favourites and that’s not easy to have. Last season Chelsea and Liverpool were not in Europe and that gave them a chance to be better and stronger than the others in the championship.”

United have so far invested £31m on central defender, Victor Lindelof, and an initial £75m on No9, Everton’s Romelu Lukaku. “We spent big money on a striker because, with strikers, you either spend [big] or you don’t get,” said Mourinho. “And we spent because of what happened to Zlatan [Ibrahimovic, who has a serious knee injury] and we cannot allow ourselves to be without an important, strong striker for the first six months of the season.”