It was expected Dembele would miss the tie against United, with the first leg to be played on April 10 and the return held at Camp Nou six days later.
But the France forward returned to training on Thursday, giving Barca a boost ahead of the visit of Atletico - who are eight points behind the leaders - at the weekend.
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 coach Diego Simeone.
Barcelona are eight points ahead of Atletico as the clash at Camp Nou approaches. The Catalan side saw their seven-game winning run come to an end on Tuesday when they drew 4-4 with Villarreal. Valverde's men are unbeaten in their last 15 matches in all competitions.
Atletico, meanwhile, have won seven and lost two of their last nine and have moved five points ahead of city rivals Real Madrid in the Spanish top-flight.
Having seen his side knocked out of the Champions League last month, Simeone has vowed his side are not giving up on their battle to win La Liga.
"We can't lose sight of the fact that, at any point in La Liga, competing is in our nature," he said. "And we want to compete for La Liga until our last breath."