It will be Mina that makes way. He's on a big salary (for us) and is going to interest a wider range of clubs in more countries because he's an established international who has language skills that will allow him to settle in quickly. By comparison, Holgate will only attract bids from PL clubs.
From our perspective, based on the premise that Koulibaly would be coming in as a starting XI player, we'd also be better off keeping Holgate than Mina. The pecking order would be:
First choice: Koulibaly & Keane
First off the bench: Godfrey
Developing: Holgate
That makes far more sense (considering salaries, ages and current levels of development) than:
First choice: Koulibaly & Mina
First off the bench: Keane
Developing: Godfrey