Roberto, a natural midfielder who also plays as a full back, was forced off in the first half of Barca's 0-0 draw at Greece's Olympiakos on Tuesday in the Champions League with a hamstring injury.
Portugal's Gomes also ended the game with a hamstring problem even though he only played the last 15 minutes.
Barca are unbeaten in all competitions so far this season and top La Liga with 28 points from 10 games while leading Champions League Group H with 10 points. But their midfield pair will miss league games with Sevilla, Leganes, Valencia and the trip to Italy's Juventus.
Gomes should return in time to face Celta Vigo on Dec. 3, with Roberto being back to face Villarreal on Dec. 10.
Ernesto Valverde's Barca side currently have seven players injured in total, including captain Andres Iniesta and record signing Ousmane Dembele, who will not be available until January.
(Reporting by Richard Martin Editing by Jeremy Gaunt.)
