Blaugrana manager Ernesto Valverde still must cut one player from his squad, meaning it is possible Messi could still miss out due to a thigh injury.
The Argentine star picked up the injury on Saturday in Barcelona's 2-2 draw with Valencia.
Though Messi played the entire match and scored twice, Valverde confirmed after the game that the 31-year-old had a "small problem" and would face tests to see if he'd be ready for Wednesday.
"We will have to wait to see what medical staff say. If he's in good shape, he'll play on Wednesday," Valverde said on Saturday.
Though Messi was included in the gameday squad, Ousmane Dembele missed out due to an ankle problem. The Blaugrana are still hopeful that the France international will be fit for Sunday's La Liga match against Athletic Club.
Dembele joins goalkeeper Jasper Cillessen, who has been ruled out for six weeks with a calf injury, on the sidelines, as well as Samuel Umtiti and Rafinha.
Earlier on Tuesday, Valverde was non-committal on his star player's status.
"If Leo is ready then he'll play, if he's not then he won't, Valverde said. "But I don't think that changes who the favourite is. If he's fit he'll be in the squad and if he's not then he won't be."
Madrid and Barcelona will meet in the second leg of the semi-final on February 27 at Santiago Bernabeu. Just three days later, the teams will meet again in a La Liga fixture.