The Argentine star picked up a thigh injury during his decisive display against Valencia on Saturday after he converted his 50th penalty in La Liga and then pulled his side level with a second-half strike, but needed treatment on the touchline shortly afterward when he pulled up with an apparent thigh injury.
Although he returned to see out the 2-2 draw with Los Che at Camp Nou, coach Ernesto Valverde confirmed after the game that Messi had a "small problem" and would face tests to see if he is available to feature against fierce rivals Madrid in midweek.
"We will have to wait to see what medical staff say. If he's in good shape, he'll play on Wednesday [against Madrid]," he said.
Goal now understands that the 31-year-old was still struggling with the injury on Monday, however, and did not take part in normal training with his Barca team-mates.
The Catalan side will assess their top scorer again on Tuesday to see if he is in good enough shape to feature against Santiago Solari's side.
While the Argentina international could be kept out of the squad altogether if he is not completely fit, Valverde could opt to name him among the substitutes in case he is needed.
Messi may be a doubt, but Valverde will welcome Ousmane Dembele back to the team for the midweek encounter.
The winger has missed Barca's last four games with an ankle injury but is back in normal training this week, and the France international will be named as part of the squad and is in contention to start in his second Clasico.
Barca are already without goalkeeper Jasper Cillessen, who has been ruled out for six weeks with a calf injury, as well as Samuel Umtiti and Rafinha.