You can also help other players to the best of your knowledge, would love to have you here on the community.
So as to the answer of your question, to upgrade a Player, you need him out of the team(not on the starting 5 nor the bench) Specifically, on ANY lineup. I see you're a relatively new player with 95 ovr ultimate, so you might have used that 91 Turgoklu in some other lineup(street, untimate, or muti year) you need that card out of every team, and it will be available for upgrades,
@TheScholarAM Thank you, you were right I checked only the line up fpr Surge/ Storm and not the other lineups for Turkoglu's upgrade. I'm glad to have your advice to resolve this game features event.