He didn't need them to win the league back then and they were clearly not yet ready to handle premier league. With Roman Abramovich you can't say you are building team for the future , It's always about win it now or sack so yeah Mourinho has no time to wait for Salah and KDB to develop.
You can also say he has Drogba and Lampard at Chelsea but so does Avram Grant , Scolari did the later won the league ? He had Eto'o and Milito at Inter but so does Rafael Benitez but did the later won the league ? He has Kane and Son but so does Poch , did the later won anything? Surprise to you is that a good manager made a world class player better. Ole has Martial , Rashford , Pogba , all the fans favorite player ( not jose's favorite ) and we are still trophyless.
So DDG scored all the goals ? We scored more goals and conceded less in that season compared to last season with martial , rashford , greenwood , maguire , awb and all fans favorite players playing regularly. DDG excellent performance were also down to Mourinho setting us up to be compact , DDG is best at being shot stopper , but not so good with the ball and distribution which means he is actually suited better with a team who "park the bus" and force opposition to shot from more save-able position than a team who dominated possession and try to play it from the back. If any DDG needs a coach like Mourinho to thrive , not the other way around.