United scored four goals, three from set-pieces, as Chris Smalling, Juan Mata, Marcus Rashford and Paul Pogba with his first goal since his return from Juventus, all found the back of the net between the 22nd and 42nd minutes at Old Trafford.
England skipper Rooney was limited to just a few minutes off the bench at the end of the match but Mourinho told Sky Sports: “Wayne Rooney is a big player for me, United and this country.”
Leicester managed a brilliant consolation goal from half-time substitute Demarai Gray but that could not stop the Foxes shipping four goals for the second time in as many league away games after doing so at Liverpool a fortnight ago.