The midfielder did not feature against Los Blancos, despite being named in the original starting XI, and he stayed on the bench throughout the match.
Ernesto Valverde said afterwards Busquets was feeling bad and had a fever, and the 31-year-old was still feeling the effects on Thursday and sat out the session.
Barcelona’s next game is at home to Alaves on Saturday, and it remains to be seen if Busquets will be fit enough to start their final La Liga match of 2019.
Ousmane Dembele and Arthur were also present in Thursday’s session as they “continued with the recovery process from their respective injuries.”
The Frenchman is back at the club after starting his rehabilitation in Qatar, while Arthur is being tipped to return to action in January.
Barca train again on Friday morning which will be their last session before their next game.