Messi was missing in action with a leg problem when Barcelona defeated Real Sociedad on penalties in the semi-final on Wednesday following a 1-1 draw in regulation time.
Barcelona’s captain has since returned to training but is still considered a doubt for the showdown at the Estadio La Cartuja.
"We will see. The player himself always has the last word," Koeman said at his pre-match news conference when asked if Messi will play.
"We will wait until Sunday morning to see how his body reacts. We are hopeful he will be available tomorrow.
"If you have the best players in your side, you always have a better chance of winning. We are talking of the number one player in the world.
"With Leo the team is stronger in terms of creativity and effectiveness, which we otherwise sometimes lack. We have a system in mind but it depends if Leo is involved or not."
Sunday’s game will present Koeman with the opportunity to win his first trophy as Barcelona manager but the former Netherlands boss is wary of the threat posed by a Bilbao side who knocked out Real Madrid to make the final.
"It is a demonstration that tomorrow everything is possible," Koeman added.
"If Athletic are as competitive as they were [against Madrid] then it can make for a complicated game. They were effective against Madrid and deserved to go through."
