Although this is kind of cheating, Mourinho had such a dramatic impact on Chelsea's story when he became manager in 2004. The Portuguese tactician only lasted for three full seasons, but Chelsea rose exponentially in prominence during his time there, and if he had managed to pull off a Champions League victory while in London, he would probably still be the manager there.
Mourinho has gone on to prove that his firing was a horrible mistake. Abromovich knows this too, and the proof is in his appointment of Andre Villas-Boas in 2011. AVB is often seen as Mourinho's protege, and the fact that Abromovich spent €15 million on him proves how costly of a mistake firing Mourinho was.
Regardless, Mourinho's work at Chelsea put the club on the map. He developed a squad that has since been in contention for domestic and European honors year after year.