Mourinho's reign at Old Trafford was ended on Tuesday with Ole Gunnar Solskjaer parachuted in as caretaker boss until the end of the season.
Former captain Rooney explained how a toxic atmosphere at United's HQ under the Portuguese coach spread further than the playing squad.
He told BT Sport: "[It was] not just the players, but the whole football club.
"I'm sure Ed Woodward was the same, the staff, the kitchen ladies, the kit men weren't enjoying it. Ole's the person [to help] straight away and I know they're happy."
And after Saturday's thumping 5-1 win over Cardiff in his first game as boss, Rooney believes Solskjaer has returned the feel-good factor to all at United.
The ex-England skipper reached out to his former team-mate when the Norwegian's appointment was announced.
Rooney said: "Once Ole got the job, I played with Ole so I sent him a message of congratulations."
He expressed to Solskjaer "my feelings of how it's been from speaking to some players and what I've felt needs to change.
"First of all, it was getting the lads enjoying playing football again. That's what they want."
Solskjaer's first fixtures as head coach offer an opportunity to get on a winning run.
Next up for United are visits from Huddersfield and Bournemouth, two clubs struggling with form, followed by a trip to Newcastle and FA Cup tie with Reading.
By then the mood in the club canteen looks likely to have lifted.
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