Look no further than Mourinho's stint at Chelsea from 2013-15. In the 2013-14 season, Mourinho led Chelsea to a Premier League trophy and a League Cup. After that successful season, Mourinho went on to get a new four-year deal that was only ripped apart by mutual consent a few months later after losing his first 9 of 16 games in the Premier League. Chelsea also had two players in their squad named Kevin De Bruyne and Mo Salah, who were never really given a chance to develop under Mourinho, and instead sold off, only to become world-class players at rival clubs. Someone else was then relied upon to pick up the pieces, and Chelsea finished that season in 10th place.
That is the type of immediate effect I expect Mourinho to have on Tottenham next season. Unlike their London rivals Arsenal and Chelsea, who have squads built around promising young players, Tottenham's designed the team to try and win now. Harry Kane, Heung Min Son, Toby Alderweireld, and Hugo Lloris are all around the age of 30 and only have a few years left to perform at their best. They have also made some good transfers, recently bringing in players like Steven Bergwijn, Tanguy Ndombele, and Giovani Lo Celso that look like they could contribute to a winning team.