Hansi Flick's side dropped points for the fifth time in six league games after Leganes' Sergio Gonzalez scored the decisive goal in Sunday's clash at Estadi Olimpic Lluis Companys.
Barcelona may have finished the weekend in top spot, but the result allowed Atletico Madrid to move level on points, while Real Madrid are just a point further back in third place.
Unfortunately for Barcelona, their disappointing weekend was made worse by the fact Yamal picked up an injury in the 1-0 defeat.
The Spain international sustained the injury in the first half but played on until the 75th minute when he was replaced by Gavi.
Barcelona confirm full extent of Yamal injury
Earlier on Monday, it was reported that the 17-year-old would undergo scans to learn the full extent of the injury issue.
Barcelona have since released a statement to confirm Yamal has suffered an ankle injury which could keep him out of action for up to a month.
The statement read: "The first team player Lamine Yamal received a blow to the right ankle during the game against CD Leganes on Sunday.
"Tests carried out on Monday have revealed that the player has a grade one injury to the anterior tibiofibular ligament in the ankle. The player is expected to be out for 3 to 4 weeks."
What games could Yamal miss?
Yamal's absence represents a major blow for Barcelona, given the fact the winger has scored six goals and provided 12 assists in 21 appearances this season.
Crucially, the Barcelona star will be unavailable for Saturday's top-of-the-table clash against title rivals Atletico Madrid.
There will then be a short winter break before Barcelona begin the new year with a Copa del Rey tie against fourth-tier Barbastro on January 4.
While Barcelona should have enough to beat Barbastro without Yamal, there is a possibility that they could be without their star man for the Spanish Super Cup.
The Blaugrana will travel to Saudi Arabia to face Athletic Bilbao on January 8 for the chance to play in the final against Real Madrid or Mallorca on January 12.
Barcelona will certainly hope to have Yamal back available for their first La Liga match of 2025 which will take place on the weekend starting January 18.
Written by
Ben Sully