"Fixtures lists are always hard at this time of year, throughout January, and with this semi-final with two top-level games, league games and the Champions League starting soon, we all need to be careful", Valverde added.
Messi set alarm bells off after sustaining a knock to his leg during Saturday's 2-2 La Liga draw with Valencia, although he was able to finish the game after being treated, with Barcelona not releasing a medical report on his condition.
Barcelona forward Ousmane Dembele is unlikely to be fit for the game, as he is still recovering from a hamstring injury. They trained with the club opening the doors to the media for a spell and everything seemed fine.
"El Clasico does not change without Messi; in a semi-final between Barca and Madrid there is no favourite, it is an open tie, we have good players, they too", he said. "We'll see the best of Real Madrid tomorrow, that's what we've seen in the last few games". "We'll see if he can complete the session, if he will show that he is in condition to play". Fatigue will obviously be a cause of major concern as the season progresses as Valverde would want a player like Messi to be at the top of his game as the business end of the season approaches. 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.
'The most important thing is whether the player tells you he is ready or not.
"They were in a completely different place then, which needs to be taken into consideration", he said of Madrid, who have won seven of their last eight matches in all competitions.