A hamstring injury suffered by the Spain Euro 2024 winner at Girona means he will be absent until after the October international break, leaving manager Hansi Flick very short on numbers in the center of the pitch.
Defender Eric Garcia was forced to play further forward when replacing Olmo on the hour mark at Montilivi on Sunday with no other midfield options available to Flick.
Fermin Lopez is not ready to return after a muscular injury, and even though Gavi is back in training after an ACL tear, he is also not available yet.
Frenkie de Jong's recovery from the injury that saw him miss Euro 2024 remains slow, while youngster Marc Bernal is out for the season after suffering a serious knee injury of his own earlier in the campaign, leaving Flick with only Pedri and Marc Casado as viable options in midfield.
The injury list piling up was reflected in Barcelona's matchday squad at the weekend, which included three teenagers, and an entire complement of under-25s.
In defence, Andreas Christensen remains out with an Achilles injury, while Ronald Araujo, who would have missed this game with suspension after his red card against Paris Saint-Germain in last season's quarter-final, is also on the treatment table with a hamstring issue.
Ansu Fati is back in training and could be part of the squad soon, which will be useful in their upcoming league games with Ferran Torres now serving a suspension after being sent off at the weekend, but he will still be available for European competitions.
On the field, Lamine Yamal will have the eyes of Europe on him again as the teenage sensation dazzled once more against Girona at the weekend.
Yamal's two goals in the 4-1 win means he has contributed to at least one goal in all five of Barca's matches so far this season, with three goals and four assists.
The 17-year-old will be seeking his first Champions League goal here, while Robert Lewandowski is closing in on a century, as the Pole is now on 94 across his career, with eight of them coming for Barcelona.
Barcelona possible starting lineup:
Ter Stegen; Kounde, Pau Cubarsi, Inigo Martinez, Balde; Eric Garcia, Pedri, Casado; Lamine Yamal, Lewandowski, Raphinha