Klopp pretty much said that today
No he didn't.
Ok, he said that he’s not the player they bought, yet. Better?
Implied that it was temporary. The essence of his point was clearly that he think he's currently adapting to the new environment and that he thinks he will eventually become a valuable player for Liverpool. No where near the 'overrated' view.
It happened to Fabinho, Klopp acknowledged it, and now he's getting better. He think it's the same case with Keita.