The 17-year-old was unable to influence proceedings as Hansi Flick's side succumbed to a fourth-minute winner from Sergio Gonzalez, meaning that the Blaugrana have won just one of their last six matches in the top flight.
Yamal was on the field for 74 minutes at Estadi Olimpic Lluis Companys, but the Euro 2024 winner was left writhing in pain on the turf inside the opening 20 following a tackle from Yvan Neyou.
Yamal was able to carry on after receiving treatment from the medical team, but the talented teenager was in visible discomfort afterwards and could not last the full 90 minutes.
According to the Daily Mail, Barcelona are fearful that Yamal sustained a sprain, and the 2007-born phenom will undergo further examinations in the coming hours to determine the severity of his issue.
Yamal at risk of missing crunch Atletico clash?
Yamal has already spent a few weeks on the sidelines with one ankle injury this season, sitting out clashes with Brest, Real Sociedad and Celta Vigo, as well as Spain's Nations League battles with Denmark and Switzerland in November.
It is believed that the attacker's new injury is also on the same side as his previous ankle concern, and he is already an early doubt for this weekend's mouthwatering La Liga battle with Atletico Madrid.
Flick's side are still top of the La Liga table despite Sunday's upset, but Atletico are now their closest challengers and are only behind on goal difference with a game in hand on Barcelona.
Diego Simeone's side are also one point clear of city rivals Real Madrid and will head into Saturday's clash on an exceptional run of form, winning each of their last 11 matches across all competitions.
Barca are already working without long-term absentees Marc-Andre ter Stegen, Marc Bernal and Andreas Christensen, while Ansu Fati is also touch-and-go for the Atletico clash due to a hamstring problem.
How long could Yamal be out for and who might replace him?
The fact that Yamal was able to momentarily play through the pain on Sunday suggests that his sprain is not too severe, although the immediate treatment and adrenaline may have played a part in that.
If the 17-year-old has indeed suffered the least severe grade of sprain, he could be fully operational again within a week or two, although even the most mild injury would still render him uncertain for the clash with Atletico.
Should Yamal not be passed fit for the showdown with Simeone's side, Ferran Torres would be his most natural replacement on the right-hand side if Flick wishes to keep the influential Raphinha on the left.
Alternatively, the versatile Dani Olmo could take up a wide role, allowing either Fermin Lopez or Gavi to come into the number 10 slot in between the Spaniard and Raphinha, but a promotion for Torres seems the most logical solution at this stage.
Yamal missing out would be a devastating blow, though, as the teenager has had a direct hand in 18 goals in 20 games this season, six of his own and 12 assists.
Written by
Ben Knapton