Chelsea legend Frank Lampard has warned Jose Mourinho that Petr Cech's summer move to Arsenal could lead to the Gunners winning the Premier League next season.
Even though the Portuguese boss said on a number of occasions that he wanted the Czech Republic international to stay at Stamford Bridge, the Blues sold him to their north London rival on 29 June in a deal, according to BBC, that is worth about £10m ($15m).
Mourinho then hailed the decision of the Blues while accepting the Chelsea legend's decision to move to a rival after losing the battle for the number one role at the club to Thibaut Courtois.
"I always said I wanted him to stay but I understand Petr needed to move on to play first-team football every week," Mourinho said to Chelsea's official website.
"It is very rare in football to make a decision like this and for that reason I am proud of my club for making it. There are not many clubs in the world big enough to be able to make that decision."
But Lampard suggests the Blues may regret their decision as his former Chelsea teammate could "make a difference" in the Premier League battle.
"Arsenal signing Petr Cech is a big deal. A lot of mates who are Arsenal fans have bemoaned their 'keeper situation for ages," the former Manchester City and Chelsea midfielder said, as quoted by the Express.
"Cech is a world-class keeper and person and can help push Arsenal into the running next season. Arsenal weren't a million miles away over the last few seasons."
Just before the goalkeeper's move from Stamford Bridge to the Emirates Stadium, Blues captain John Terry warned Chelsea that should Cech join Arsenal, he "will save them 12 to 15 points a season".
And Lampard has reiterated the same idea, claiming that even though the Blues and Manchester City are still the favourites for the Premier League title, Cech can change that scenario.
He said: "They are always challenging before falling just short but Cech going there, as John Terry said recently, will save them points. Manchester City and Chelsea will be challenging for the title once again. They remain favourites but Petr will make a difference."