Dembele sustained a torn hamstring after coming off the bench in Barca's 5-1 last-16 second-leg win over Lyon, with Ernesto Valverde accepting he knew he was risking the winger's fitness.
It was expected Dembele would miss the tie against United, with the first leg to be played on 10 April and the return held at Camp Nou six days later.
But the France forward returned to training on Friday (AEDT), giving Barca a boost ahead of the visit of Atleti at the weekend.
Atletico has its own injury issues with Alvaro Morata suffering a sprained ankle in the LaLiga victory over Girona this week.
With Diego Costa also a doubt due to a muscle problem and Thomas Lemar not expected to be fit, Antoine Griezmann and Nikola Kalinic may be the only attackers available to Atleti manager Diego Simeone.