This is true to an extent. Mourinho is cool when the results are going well, he has a step to his swagger, calls someone a loser and in general looks amazing. Its when the results don't go well coupled with having a rival who would bring out his inferiority complex that you see the worst in him, the whining, moaning, abusing physios, fighting assistants etc.
Right now his rep is at his absolute lowest in his career, so if he starts with us and loses a few early games, it brings back a ton of demons and people start wondering if he's truly lost it and its a continuation of his downfall from his last Chelsea season. Then you have Pep next door, doing well, playing great football and Mourinho will start picking fights within the club and in general turn into a pest. The team morale could take a big hit and not recover, the club is already facing its own low point since Ferguson retired.
Mourinho should have taken over the squad that walked the league and won us the champions league. Right now we need a more organic manager than Jose.