Replacement level: how many wins do injuries cost?

Replacement level can be a bit difficult to define in hockey. Unlike baseball, where Derek Jeter gets hurt and all his playing time goes to a guy who has yet to prove he can hit AAA pitching, an NHL player's minutes get spread around to other NHL players (aka his teammates) while he's hurt. So the Yankees can see the difference between Jeter and true replacement level, while an NHL team might shift every line and give nine low-leverage minutes to a guy who only dresses half the time, obscuring the difference between top players and the 21st guy on the roster.

I'm not going to try to compute replacement level here. Instead, I want to look at how many wins it costs a team when one of their top players is injured. For the last five seasons, I looked at forwards who averaged 19 minutes or more per game and defensemen who averaged 21 minutes or more - and who played between 52 and 74 games with a single team. This roughly corresponds to a team's top two forwards and top two defensemen. I compared their team's record with them in the lineup to its record with them out of the lineup. Here are the results:

Forwards (~19 mins TOI or more)

 Games Games Pts
 Played Wpct* Missed Wpct Delta/82GP
Total 6540 0.502 1619 0.484 3.07
2007-08 1292 0.508 348 0.487 3.39
2006-07 1331 0.491 309 0.479 2.03
2005-06 1344 0.510 296 0.483 4.48
2003-04 1240 0.505 360 0.474 5.19
2002-03 1300 0.492 296 0.441 8.37

Defensemen (~21 mins TOI or more)

 Games Games Pts
 Played Wpct Missed Wpct Delta/82GP
Total 6610 0.498 1546 0.447 8.44
2007-08 1325 0.486 315 0.443 7.08
2006-07 1322 0.505 318 0.447 9.64
2005-06 1318 0.503 322 0.429 12.27
2003-04 1300 0.492 296 0.441 8.37
2002-03 1345 0.505 295 0.478 4.47

The overall winning percentage when players were in the line-up was close to .500 for both datasets, which we would expect since playing time is evenly distributed across good and bad teams. And for both forwards and defensemen, there was a clear difference in winning percentage when these players were out of the lineup. However, the difference was much greater for defensemen overall, and in four out of five seasons for which we have ice time data. Teams can expect to lose four extra games if one of their top two blueliners goes down for the season, while losing a first-line forward appears to cost just one and a half losses. The error bars on these estimates are large, but it's clear that losing a defenseman is a bigger deal than losing a forward.

* - Winning percentage was calculated assuming SO and OT games were ties.