Sergio Busquets had stomach problems during Barcelona's game against Slavia Prague which left him dizzy and forced him to be taken off in the second half.
The Spain midfielder was replaced by Ivan Rakitic in the 68th minute as Camp Nou as Ernesto Valverde's side were held to a surprise goalless draw in the Champions League by the Czechs.
Busquets will undergo further tests in the coming hours and it's not been ruled out that he sits out the weekend's game against Celta Vigo in La Liga.
He remains a regular in Robert Moreno's Spain squad and is expected to be in the squad for upcoming qualifiers against Malta and Romania, although La Roja have already sealed their place in next summer's European Championships.
Jordi Alba (hamstring) and Luis Suarez (calf) are also major doubts for Saturday's match against Celta.