Jose Mourinho says a lack of signings left Chelsea's players in the comfort zone and caused the woeful Premier League title defence that prompted his departure.
Chelsea were dominant last season as Mourinho won his third and the club's fourth Premier League crown, the Blues topping the table by eight points from nearest rivals Manchester City.
However, their attempt to retain the championship has been hugely disappointing and Mourinho parted company with the club for the second time in December after the team lost nine of their opening 16 league matches, while rumours surfaced of dressing room discontent.
The Portuguese believes Chelsea's failure in the transfer market was a key factor, with Everton defender John Stones among the targets they missed out on during the close-season.
And the former Porto, Inter and Real Madrid coach feels that a lack of investment contributed to Chelsea's stars underperforming.
"If you are in a club that wins, if you want to win again, you have to create instability in the winners," he told Sky Sports.
"To do this, you have to make them doubt, you have to buy new players, you have to take them outside their comfort zone because if you win, there is a normal tendency to go to a comfort zone."
Mourinho won La Liga during his time in charge of Madrid and while he acknowledged it was difficult to overcome fellow superpower Barcelona, he also criticised the lack of competitive balance in matches against other Spanish clubs.
"I always say that I need competition and I need competition every week and in Spain, I was at an amazing club but I had four matches a year: Barcelona v Real Madrid, Real Madrid v Barcelona and after that we'd win 4-0, 5-0, 4-1, 6-1," he said.
"It was difficult to win because two monsters are together, but it was easy to win matches. Difficult to win the league because you are competing against another team that also win and win and win like you do, so in the end, I was champion in Spain with 100 points and I lost a league in Spain with 91 points.
"In England, you can win the leagues with 75 points, maybe less, so I needed competition."