Chelsea's defensive solidity (or perceived solidity) is down to Mourinho's legacy at their club. For all intents and purposes Chelsea still play like a Mourinho team - efficient, clinical, and solid. What they lack is inventiveness. They do not create lots of chances, they are merely good at finishing the few chances they get.
Conte is really a lucky man, he gets a lot of credit for the foundations that Mourinho built.
Give Mourinho some time. One more summer window to let him get the players he needs. And the good news is that UTD is currently undergoing a very exciting project, blending Jose's philosophy (efficient play, solidity, winning mentality, hustle) with the club's own philosophy (attacking football, creating lots of chances, young players).
When the two philosophies merge and blend with each other we could very well experience something very special taking place. I am extremely confident that next year UTD will surpass Chelsea because Jose will take on all of the Chelsea-like qualities and add the UTD-like qualities of flair, risk taking, and inventiveness. Best of both worlds. I am extremely sure we will be champions under Mourinho.