You are right CAE82, it can increase determination.
i started a new save with PSG, bought a player (Mamadou Diakhon) with low determination (7) and he is now at 16. I used Mbappe to welcome him at the club.
I even had him up to 18 with an other save.
I uploaded the save for this example in the cloud. "PSG Nov 10.fm" and "PSG Oct 24" when he was 10 determination.
"PSG Nov 3" is the save where he moved up to 18.