The former Blues defemder returned to Stamford Bridge with Manchester City on the weekend, scoring in their 3-0 win.
Cole told The Dressing Room Podcast: “I’m looking at him and I’m thinking: how has this kid got a Manchester City shirt on? The team should have been built around him five years ago.
“Whoever has let him out the door at Chelsea… if I’m the owner, I’m going, ‘Right, who’s decision, or what collective of people’s decision, was it to think it was a good idea to let this kid leave the club?’.
“Everyone would go, ‘It wasn’t me, it wasn’t me!’… unless you find out who is making these decisions, these decisions will continue to happen and you’ll keep losing players like that. You need to do an appraisal on it.”
He added: “For me, the big moment in the game from a Chelsea perspective, and from a Manchester City perspective, was when Marc Guehi scored the goal.
“It was a great finish but I look at where Chelsea are as a club, desperately trying to find a centre-half, spending hundreds of millions of pounds… you had a kid in the building.
“He came over and spoke to us and he’s so assured, he’s a captain. You see him play and he’s calm.
“He came from Chelsea from the age of six, we sold him to Crystal Palace and now he’s at Manchester City and he looks like he’s been there for 20 years.”