Vatuvei, 25, suffered damage to his medial ligament in the early stages of the Warriors’ home defeat to Parramatta on Saturday.
The New Zealand test player limped off after being hit by Eels winger Etuate Uaisele in a torpedo-like tackle.
Although Vatuvei will not need surgery, he is unlikely to recover before round 7.
“I’m down on myself but that’s how it goes in this game sometimes. There’s nothing I can do about it,” said Vatuvei.
“I just have to look after the injury and hopefully come back stronger. In the meantime, I’ll be there doing the best I can to support the boys.”
Coach Ivan Cleary was relieved the injury wasn’t more serious.
“It could have been quite a bit worse. Thankfully it isn’t,” he said.
“Manu’s staying positive and that’s his nature. Obviously it’s disappointing for him and for us but we now have to move on and adjust to being without him for a while.”
The Warriors will be hoping to bounce back from the defeat when they take on Wests Tigers this weekend.