Messi recieved a blow to the leg off Toni Lato during their 2-2 draw with Valencia on Saturday night and it left his availability up in the air as the muscular discomfort continued after the game and into the next couple of days.
Nonetheless, a fit and available Messi is clearly preferable for the La Liga leaders who are giving their captain the time he needs to to prove his fitness.
Forward Ousmane Dembele is also unlikely to be fit for the first leg at the Nou Camp, as he is still recovering from a hamstring injury.
"There is a training session and we will see how he is, whether he can do it and whether he is in the right condition. We'll see the best of Real Madrid tomorrow, that's what we've seen in the last few games".
Asked whether Vinicius' recent displays made him undroppable, Solari replied more shortly that: "Nobody is undroppable in football".
"In a semifinal between Barca and Madrid there's no favourite". Against Sevilla, they went all out last week to claim a 6-1 win and turn a first leg's deficit in the Copa del Rey, whereas over the weekend, they had to come from two goals down to pick up a point against a resolute Valencia side. "If Messi's ready he will play, if he's not he won't".
"When we prepare the game obviously we have the reference of the players and how they performed [in the 5-1], but there was a different morale [at Madrid] at that time, that counts for a lot".
"We put in a massive effort, it's not easy to play against Valencia who were feeling more confident after their recent results".
The draw presented Atletico Madrid, playing on Sunday, the chance to close the gap at the top to three points.
Real Madrid: Improvement is what's on this team's mind, showing something better than in the first match of the season.