The former Blancos man feels it is the shot-stopper's performances the Portuguese coach is unhappy with rather than it being a personal issue, and says Europe is now the priority
The reigning La Liga champions are 16 points behind perennial rivals Barcelona with the winter break under way, and sit third behind Tito Vilanova’s league-leading side and Atletico Madrid.
Buyo criticised the Madrid squad for their performances this season and insisted that there are too many at Santiago Bernabeu that are not up to the task, while claiming that the Spain international is not necessarily in the head coach’s bad books.
"I refuse to think that Mourinho has anything personal against Casillas," he told Marca.
"If we look at performance, the majority of players are not up to the level the supporters and shareholders would wish. Something is going wrong.
"It would be unfair to point the finger at Iker for the Malaga substitution. I am not going to name names, but there are many who are not up to the mark.
"Success in Europe has covered them with much praise, but it has had an effect on the drop in form and confidence of Real Madrid."
The former Blancos shot-stopper believes that Barcelona have got La Liga in the bag already, and says that Mourinho must now focus his efforts on Champions League success.
Buyo, 54, added: "If [Barca] lose this league title to Real Madrid it would be a disaster, but I cannot believe [they] would lose seven games in the league.
"If you lose the league to Barca there is no need to tear up your kit. You have to go down fighting and working hard. If they do that then they will be good for the Champions League."