The Red Devils slumped to their fifth defeat of the season to leave themselves 16 points behind leaders and local rivals Man City and only two clear of third-placed Liverpool.
Barton believes that the pairing of Mourinho and executive vice-chairman Ed Woodward are leading the club down a dangerous path, describing them as "lotto louts".
"I don't think [Mourinho] is the fit for them. Everytime there is a negative performance, a negative result, the fans [say] 'this isn't the United way, we don't play like this, he is not our kind of manager'," Barton told talkSPORT.
"I don't think he has done anything to endear himself to the Man United faithful. I don't think he is a perfect fit. Ed Woodward is a space cade. The fact he has got himself involved in a Dutch auction and given [Alexis] Sanchez the money he has given Sanchez and I think they pay over the top all the time for players all the time.
"When David Gill and that regime was there, the attraction of playing for Man United was the big thing, whereas now they are just outspending. They are like Michael Carroll and the Lotto louts, outspending everybody. If they don't sort it out quickly it is going to end up in tears for them because they are they are so far behind the team on the opposite side of the city."
United will now turn their attention to FA Cup duties, where they will take on Huddersfield Town in the fifth round on Sunday.