Spain head coach Luis Enrique hopes defender Dani Carvajal can overcome a bout of flu after sitting out Monday's training session at Doha University.
The Real Madrid defender's absence comes as regular starting striker Álvaro Morata returned to practice after his own bout with the flu.
Central defender Hugo Guillamón, nursing a knee issue, trained separately and was unlikely to be available for Wednesday's opening match against Costa Rica.
Germany and Japan are the other teams in Group E.
Luis Enrique - who led the Reds to the semi-finals of the Euros last year where they were eliminated by Italy on penalties - has picked 14 players aged 25 or under, and the average age of the squad is the third youngest among the 32 teams in Qatar.
"We are a team with a lot ahead of us, we are very young," striker Ferran Torres told reporters on Sunday.
Asked whether he felt the team was obliged to prove themselves by finishing in a strong position in the tournament, Torres stressed Spain remained a work in progress.
"We're not obliged to finish in any place. We will simply play the best we can to get results and if we can we will keep going. If we don't get the results, we will go out with our heads held high and keep working. We are young."