The Blues won the Premier League title in Antonio Conte’s first season in charge at Stamford Bridge as they won the title for the second time in three years.
Arsenal, meanwhile, won the FA Cup but endured a disappointing campaign in the Premier League as they ended up fifth in the table and missed out on Champions League qualification.
Manchester United enjoyed a relatively successful first campaign under Mourinho as they won the EFL Cup and Europa League to secure a return to the Champions League.
However, the Red Devils came up short in the Premier League as they ended up in sixth place in the table.
Speaking at a news conference in LA on Friday, Mourinho said: “First of all I think after the champion, Chelsea, I think Manchester United is the second club that we can say most successful last season and Arsenal the third.
“For me that’s absolutely clear, to finish second, third or fourth is like being… because there is only one winner.
“Arsenal won a competition, we won two-and-a-half, I like to say, so the other clubs won nothing so they have to improve.”
Manchester United have signed Victor Lindelof and Romelu Lukaku so far this summer as Mourinho looks to bolster his squad ahead of his second campaign in charge at Old Trafford.
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