The 26-year-old, who is looking to revive his career in Spain, may not be fit to face Pep Guardiola's men at the weekend
The former Juventus playmaker trained lightly on Thursday but still does not look fit enough to be on the bench for the capital club at Camp Nou, according to Marca.
Diego practised alone during the training session, working with the ball but did not play at full speed.
The midfielder has appeared in two league contests as a substitute and also participated in the club's 2-0 win over Celtic in the Europa League.
Atletico sit in eighth place in the Liga table with seven points, three behind leaders Valencia.