People have been very hard on him. For the at best 9th guy on our depth chart forced to play a #4 role, he has been good. He's had to jump 5 spots up the chart. Krug and Mikkola are out with Covid. Perunovich was more expensive and not able to be called up. Walman has been bad and been short-iced. Being a LHD, he has also gotten more time than Bortuzzo. Its arguable that entering the season Santini was ahead of him too, but he passed him up in the AHL as well. When Krug comes back, he'll be sent down or demoted to Walman's role. That is more fitting for what he brings. But he has been absolutely fine as a fill in.
Rosen has made some really good passes that surprised me, and our own guys. He hit ROR with the most beautiful pass that ROR fumbled a couple games ago and missed the net. It should have been an easy wide open goal. I have seen similar plays a few times. Nothing has come of them though, so he doesn't get any credit if they don't make the highlight reel.
His defense has been fine to good. He is definitely sheltered but he has the best xGA/60 of any Blues D and best HDCA/60 of any Blue at all. So you really cannot ask a ton more from him given his role defensively. He basically has slid into Krug's sheltering and done better defensively from a shot supression/HDC supression standpoint. He has had some terrible on-ice SV%, so his GA aren't where Krug's are. You can't blame him for that. I don't recall any outright flubs where he actually caused the goal directly.
So overall, for a guy who started the season as a #3 in the AHL, to come in and lead the team in xGA/60 while creating a few prime chances offensively, that's a huge win for us.