Lookman also pointed out that he had dealt with many challenges during his time at the club.
Liverpool had initially shown interest in former player and Bayer Leverkusen manager Xabi Alonso.
He will train with Chelsea for the next few weeks, after which the West London-based club is expected to decide whether to sign him.
“Germany international Kai Havertz has joined us from Chelsea on a long-term contract,” the club said in a statement.
The action left the French club furious and has now decided to punish the player.
Chelsea are expected to reappoint the former club’s captain and manager, Frank Lampard as their new caretaker manager.
Chelsea, who have struggled to hit the ground running this season, are looking to bolster their attacking options.
The medical tests secretly took place in Germany some weeks ago.
Discussions took place between the Chelsea hierarchy and Potter’s representatives into Thursday morning.
Costa is reportedly set to undergo his medical tests in England on Tuesday.
