After this collision late in the first period of the Leafs - Panthers game on Monday night:

Frederik Andersen finished the period, but did not start the second.

And it was not clear at that point if Andersen was just getting his equipment back on after having something looked at or if it was a more serious issue.

And the (somewhat confusing) verdict is:

He seems to catch his foot or leg, not anything on his upper body.

The internet is speculating that an earlier glancing collision with Mark Pysyk’s butt is the culprit. I went back and watched on the overhead camera, which remains on the goalie while the broadcast is showing replays, and I don’t think that was it. It’s not impossible, but during the above collision, you can see from this not great resolution image:

That Andersen’s head is tucked under during the collision, and he could have strained his neck and/or hurt his head/shoulder there.