Blues chiefs this week took the unusual step of issuing a public vote of confidence in Mourinho, who has led the team to just eight points from their first eight Premier League matches this season.
The Portuguese brought the title back to Stamford Bridge in May, two years after returning to west London, but has never managed a club for more than three successive full seasons.
“Mou is a great coach but, after a year and a half, he ruins his players,” Capello said. “They are also psychologically unable to give him what he wants. His cycles tend to last around this period of time.”
Mourinho, who has won three league titles with Chelsea, reacted testily earlier this season to suggestions that his appointments inevitably lacked longevity, calling them “stupid”.
Capello, sacked by Russia in July, highlighted the loss of form of defender Branislav Ivanovic and the absence through injury of goalkeeper Thibaut Courtois as key factors in Chelsea’s slump.
“They don’t have any rhythm, any aggression,” the Italian added. “[Branislav] Ivanovic is not his usual self and the loss of Courtois is a huge blow because lots of the goals Chelsea have conceded would not have gone in had they had their No1 goalkeeper.”
Chelsea said on Monday that Mourinho still had the club’s “full support” but warned that results would have to improve and that he should not expect funds to sign players in the January transfer window.