Just for sake of completeness I'll do the Semin WOWY including Eric Belanger and the Caps' defensemen: Mike Green, Jeff Schultz, Joe Corvo, Brian Pothier, Shaone Morrisonn, Milan Jurcina, John Erskine, Tom Poti, Tyler Sloan, and John Carlson.
Note that the first player's column in the second chart is Semin himself.
Delta1 is how much better the player's Corsi% is with Semin than without Semin. Delta2 is how much better Semin's Corsi% is with the player than without the player.
I made a mistake calculating Delta1 and Delta2 for the forwards, so I've included that data again as well. I threw in Belanger to the far right, though I wouldn't put too much stock into his numbers because of how much he was shuffled around.
Semin is a whole lot better with Ovechkin, Backstrom, Morrison, Green, and Carlson, and Semin makes Green, Carlson, Morrison, and Belanger a lot better. He's a whole lot worse with Morrisonn, Sloan, and Fleischmann. It looks like Flash and Semin have anti-chemistry. Clearly, the Caps need someone else to play with Alexander Semin.
Well, since Marcus Johansson probably won't be able to step in and play second-line center, I quickly looked up Mathieu Perreault's numbers, again using Vic Ferrari's great script. His Fenwick% was .497 and his Corsi% was .509 in 21 games played, so he was about a break-even player. But his teammates in general were pretty poor and he did alright for himself.
I'll also throw out that Perreault went 16-34-50 in 56 AHL games, a 23-50-73 82 game pace with Hershey, giving him an NHLE of 0.41 x 73 = 30 pts per 82 games. It looks though like Perreault was stuck on the depth chart behind Keith Aucoin and possible Kyle Wilson, and should make the jump to a top-6 center rather than a bottom-6, so we can add to this. Throw in PP time and "playing on the Capitals" factor and maybe we're looking at 45 points. He looks better if we take his 2.17 pts/60 and give him 18 mins of ice time a night, thus getting .651 pts/game and 53 points over 82 games, and that doesn't even factor in that some of those 18 minutes will be power play time.
Still, not certain enough for me. Perreault was a streaky player in his stints with the Capitals and a few fewer points would paint an entirely different picture. Perreault looks like he's worth giving a shot, and can certainly change my mind depending on how he starts the season, but McPhee should definitely be investigating other options for center, especially players that play like Backstrom and Morrison. In terms of defensemen, I'm not quite sure why Ovechkin and Semin show different trends. I guess simply getting a good defenseman is enough.