Club captain Messi has only started one game this season in all competitions due to calf and thigh injuries respectively.
Dembele has also been included in a 20-man squad named by coach Ernesto Valverde, after a minor knock, but teenage prodigy Ansu Fati remains out with a knee problem.
Defenders Samuel Umtiti and Jordi Alba are still sidelined and will miss Wednesday's Group F game at Camp Nou.
Inter have been dealt a blow with head coach Antonio Conte confirming in a news conference that striker Romelu Lukaku has been ruled out.
Lukaku's absence could open the door for former Barca forward Alexis Sanchez to start for the Nerazzurri, who drew their opener at Slavia Prague.