Keita was stunned by the racist abuse Koulibaly suffered last week against Inter.
He said, "I consider Koulibaly to be my brother: we play for the same National Team, and I tell him the things that I want to say personally on the phone. Such things most certainly should not be happening in our times: football is wonderful and doesn't need bad moments and insults tainting it.
"Football should be enjoyed, it provides hope and brings joy to children. We all have to do better and work together to create a better world."