The Paris Saint-Germain hitman was on trial with the Premier League side back in 2012 at the tender age of 12.
Mbappe impressed the west London club, but not enough to be handed a contract, though they did offer him a second trial.
The France internationalâ€™s mother didnâ€™t take too kindly to the clubâ€™s decision and pulled the plug on the second trial, which could have resulted in the Blues landing one of the best players on the planet.
And according to Boga, Mbappe wasnâ€™t offered a deal because he didnâ€™t track back enough.
“Defensively, he wasn’t there yet,â€ť Boga toldÂ GoalÂ when discussing Mbappeâ€™s trial at Chelseaâ€™s Cobham training ground.
â€śWhen he got the ball, he was unbelievable, but without the ball, he didn’t work a lot.
“Chelsea wanted to see him again to see how he could improve on this. I think that’s why they asked him back.
â€śHe did skillful things and dribbled well.
â€śSo, maybe the things they wanted to see in the second trial was improved work on the defensive side.
â€śThey should have signed him. To be fair, though, it is easy to say that now.
â€śBut, at the time, Chelsea had a good squad.â€ť
Chelseaâ€™s loss proved to be Monacoâ€™s gain as the Frenchman scored 27 goals in three seasons at the Stade Louis II.