Dembele was the star of the show until being forced off in the 69th minute, after a tangle with Leganes' Mikel Vesga, and Barca announced on their official Twitter account that he will undergo evaluation on Monday.
The France international had dazzled in the first half and opened the scoring with a deserved goal, turning in from 12 yards after collecting Jordi Alba's pinpoint cut-back.
But he suffered the innocuous-looking injury with just under 30 minutes to go – soon after Leganes equalised – and was unable to continue, with Malcom replacing him.
Barca went on to win thanks to Lionel Messi's heroics, their talisman playing a part in Luis Suarez's goal before then rounding things off himself late on.
Team-mate Sergio Busquets was particularly impressed with Dembele and is hopeful the injury will not turn out to be too serious.
Speaking at full-time, Busquets told Movistar: "I think recently he has been playing really well. After all of the debate and the controversy, I think he's become more confident.
"He's a brilliant player, he's really fast, he can unbalance rivals, score goals, and he was very unlucky with the injury. Let's hope he'll be okay as quickly as possible."