The Argentine didn't feature in the semi-final against Real Sociedad as he continues to find his fitness after a couple of niggling problems.
"Yesterday, he trained on his own and today, we'll see how he is to start with," Koeman said in his pre-match press conference.
"He'll have the final say. He knows his body. We have to wait and see how his niggles progress. We're hopeful that he'll play. If you have the best [players], you have a better chance of winning, and even more so if you have the best player in the world.
"The team is stronger with Leo in it, thanks to his creativity and efficiency - something that we sometimes lack. But if he's not there, another [player] will play and we'll play our game the same without him as with him.
"He has to be 100 percent. It's not the final game of the season, though the decision is his."
Sunday presents Barcelona with the chance to win their first trophy since 2019 as they take on Athletic in the Supercopa final.
"It's important because it's a final and a trophy," Koeman said on the match. "In order to show that we're on track, it's important to win things.
"We know it's not the most important trophy we're playing for, but it's a trophy. Winning things allows you to say that you're on the right track. One game isn't so decisive, but it gives you confidence and it helps you to show that you've improved.
"We can show that we're fighting. We're very focused. Our opponents will be at their best. You don't need to motivate [the players] for a final. We all have to be relaxed and confident and continue the plan that we have.
"There are so many reasons to win that you don't need motivation. We don't have to put more pressure on [the players].