Paul Merson is stunned Chelsea allowed forward Willian to sign for rivals Arsenal after he shone on his debut for the Gunners.
Willian joined Arsenal on a three-year deal last month after the expiration of his contract at Chelsea, who had offered an extension but were not prepared to match what was on the table from Mikel Arteta's side.
The 32-year-old made a magnificent start to his Arsenal career on Saturday, racking up two assists in a comfortable 3-0 Premier League win away to promoted Fulham.
Former Arsenal star Merson feels Chelsea have made a big mistake, saying there is no guarantee manager Frank Lampard will be afforded the time to watch the likes of Kai Havertz reach his peak.
"I cannot believe Chelsea let Willian go," Merson said to Sky Sports.
"I know they have gone out and get special players, but there is nothing to say they are going to hit the ground running.
"In Willian, you have got someone there who is ready-made. I like who they have signed, but they cost a lot of money, and their wages are probably the same as Willian.
"Frank Lampard has now. He has not got four years. That's management.
"I'm not being horrible. Even if Frank wins the league this year, it does not mean he is going to be there in four years' time! We know that, and how it changes.
"Who is to say this is not going to take time for Chelsea? And I don't think they've got time at the moment."
Despite liking Arteta's methods and Arsenal's performance at Fulham, Merson thinks his former club are unlikely to be battling it out with the likes Liverpool, the Manchester clubs and Chelsea.
"It was a great result for Arsenal," he said. "These are the dangerous games.
"You don't want to play promoted teams when they come up, you want to play them in January when it's hit home how hard it is, and they have a couple injuries.
"Gabriel makes his debut, is solid, and scores a goal. It was a good performance. I like Arteta, you can see what he's trying to do, playing a certain way, as a team and with a high tempo.
"I like what they are doing, but top four? I just think the gulf is too big.
"There's three or four leagues within the Premier League. I think the top four is the top four in their own league, then you have got your Arsenal, Tottenham and maybe even Everton now with Leicester.
"Then you have probably got another little league in mid-table before you get to the lower ends. It's quite interesting, and that's why it's the best league in the world."