Dembele has not featured for Barca since picking up a muscle injury in the Champions League clash at home to Borussia Dortmund in November and then suffering a setback in training as he closed in on a return in February.
The Frenchman was ruled out for six months with a hamstring injury and was ineligible for LaLiga after Barca signed striker Martin Braithwaite in an emergency transfer outside the transfer window.
Barcelona were back in training on Tuesday after a few days off at the end of LaLiga and the 23-year-old took part in the session with light work as he steps up his fitness following a long layoff.
Dembele is registered for the Champions League, but Barcelona are not prepared to take any risks with a player who has been sidelined for a total of eight months and he looks unlikely to play any part against Napoli on August 8.
Antoine Griezmann and Clement Lenglet both trained on Tuesday after their respective injuries and are expected to be fit for their team's last-16 second-leg game at Camp Nou.