He appeared to suffer muscle discomfort after scoring in Sunday's 4-1 league win at Girona and was substituted in the 61st minute.
Olmo's injury is a blow to manager Hansi Flick's side as they begin their European Champions League campaign on Thursday, coming off a perfect start to their domestic league campaign so far.
The Spain international, who joined from RB Leipzig for 55 million euros (£46.4 million), has scored in all three of his appearances for Barcelona.
The Catalan club face AS Monaco at the Stade Louis II on Thursday before Sunday's La Liga trip to Villarreal.