Alexis, a big-money arrival from Udinese this summer, was carried off on a stretcher during the first half of the 2-2 draw in San Sebastian and Barca have confirmed the Chilean is facing a lengthy lay-off.
He is expected to miss the next 13 games with Barca pencilling in the clash with Real Zaragoza on the weekend of November 19/20 as a date for his possible return.
His absence will be a blow for a Barca side still reeling from throwing away a two-goal lead at the Estadio de Anoeta.
Two goals in as many minutes from Xavi and Cesc Fabregas put Barca 2-0 up inside the opening quarter-hour and they appeared to be cruising.
But Imanol Agirretxe's header changed all that and Antoine Griezmann netted a bizarre equaliser - making good on his pre-match promise to prove his commitment to the club after the collapse of his longed-for summer move to Atletico Madrid.
Barca midfielder Sergio Busquets spoke of the need to "correct the mistakes".
He said: "Last year we also began with problems, losing to Hercules, but it's no good worrying about it.
"We have to take note of it, correct the mistakes, and think from about the Champions League and AC Milan (on Tuesday night).
"At half-time we didn't think at all that the game was won, and it turned out like that.
"It's a fact that away from home winning is always tough and will get increasingly tougher. We haven't had the chance to talk about what happened yesterday [Saturday] but we will."
Most of the Barca squad were away on international duty last week, and Spain international Busquets believes that may have been a contributing factor.
He added: "It's always tricky. You come back from another setting, another team, and different team-mates.
"It's not easy to get back to what we do well, but we can't let it happen again."