The Liga champions - who sealed victory on Wednesday at Camp Nou courtesy of a Neymar brace and a goal from Lionel Messi - have confirmed that Valdes tore the anterior cruciate ligament in his right knee after landing awkwardly when making a routine save from a free-kick midway through the first half.
With the Spain goalkeeper having already revealed that he will leave the club at the end of the season when his current contract expires, Valdes appears set for a lengthy spell on the sidelines and is facing the prospect of missing the FIFA World Cup in Brazil later this year.
Valdes has played over 500 times for Barca since making his breakthrough over a decade ago and has helped the club win six La Liga titles and the UEFA Champions League three times.
Having been taken off on a stretcher, the 32-year-old then went to hospital before the full extent of the injury became clear and head coach Gerardo Martino has rued it as a "heavy blow".
"We've been through a lot this season," he said.
"I thought everything that could happen had happened already. Obviously I was wrong. It's a heavy blow to Victor. We'll all try to overcome it.
"The group is hit hard but will continue to fight and overcome this. I don't want any excuses. We have everything at hand.
"I haven't seen Victor, nor have I talked to him. When (Valdes' replacement, Jose) Pinto went down I thought that's all we were missing.
"(But) I'd rule out signing another goalkeeper unless we suffer another misfortune."
Barcelona sporting director Andoni Zubizarreta echoed Martino's sentiments, claiming the win as an "expensive" one.
"It’s very bad luck," he added. "This was an expensive three points. We'll have to be more together in adversity.
"We saw that in the second half, Pinto was extraordinary. But we feel very bad. This victory has cost us a lot. We all feel pain."
