I'm pretty sure this didn't come out of nowhere, he has probably had knee problems throughout the season. Which could explain his drop-off in play, i mean previous two seasons he really looked like a 1#Dman, not in the elite tier but he was certainly a 1#D and this season he's looked more like a 2#D.
Knee surgery is pretty serious though, i don't know too much about it. Maybe scoping isn't that bad, but can't knee surgery set you back quite a bit? 2 weeks seems very optimistic even 4 seems optimistic, but who knows.
Worth noting is that we haven't been without kronwall for a longer stretch since 09-10, he's played pretty much full seasons since Lidström left. So this could really expose our defense imo. I hope Smith pulls it together, if not call-up XO asap he deserves it.