Messi was also missing from Wednesday's session and Barca initially played down reports of an adductor problem, but decided to clarify his absence after he failed to reappear following Thursday's day off.
"Barça captain Leo Messi has a minor right quadriceps injury and trained in isolation, doing specific exercises to avoid unnecessary risks with just eight days remaining before Barça finally get back to playing matches," the Catalan club said on their website.
"He should be able to rejoin his team-mates in a few days’ time."
LaLiga returns on Thursday with the Seville derby and Barcelona's first match is away to Mallorca next Saturday.
Messi, who also missed the start of the season through injury, must now be a big doubt for the Catalans' opener and perhaps also their home game against Leganes three days later.