It's probably 6-8 weeks, unless it was a pretty severe tear. But Klopp doesn't want to put a timetable on and just wants to let him recover at whatever pace, rather than putting pressure on him to return. Which is fair enough, we don't want to put him out when he's not fully healed and have a more severe repeat of this. It's a long season, we need him for as much of it as possible.
It's a bit mental that Adrian is now in the position he's in after a summer not attached to a club until days before the window shut, and he's got big shoes (gloves) to fill... we know he's not as good as Alisson - he knows it too. We just have to keep working as a unit and doing the best we can in this bad situation and hope that he does a good job while also hoping that Alisson is ready to go ASAP. I've seen Adrian have good games against us, I've also seen him get utterly rocked by us - he seems like a keeper that's capable of good moments and capable of moments of madness... which we're a bit used to. I hope the crowd gets behind them, he got a good reception for just hopping in, the squad are now trying to big him up. If he's full of confidence and can fill in for a... month or two admirably that won't fuck us up too badly.
I'm just relieved it's not an Achilles issue, which'd keep him out for the whole season. It's shit, but it could have been much worse. And we've seen also with Sane, who walked off the pitch against us when he'd torn his ACL... it can be a lot worse, even when it doesn't seem too bad initially because the player feels strong enough to walk.