Thornton has historically been deployed much differently than those three players but I'd love to know which matchups he's lost in the playoffs on the basis of shots or scoring chances. When it comes to goals, sure, but that's largely because Nabokov and Niemi were postseason trainwrecks for years on end.
Thornton lost the following matchups in 5V5 GF:
Brad Richards 2008 1-2 (40 minutes)*
Ryan Getzlaf 2009 2-5 (54 minutes)*
Paul Stastny 2010 1-4 (52 minutes)*
Dave Bolland 2010 1-2 (44 minutes) *
Zetterberg 2011 1-4 (65 minutes)*
Kesler 2011 0-1 (34 minutes)
Kopitar 2014 0-3 (29 minutes)*
Crosby 2016 0-2 (42 minutes)*
Asterisk means he lost in at least one of CF%, FF%, SF%, and SCF%. Red means he lost in at least three of CF% FF%, SF%, and SCF%. Bolded means he lost in every single one of CF%, FF%, SF%, and SCF%.
Thornton was on the ice for 6 5V5 goals in 360 minutes of 5V5 hockey in these matchups. That’s an average 5V5 GF/60 of exactly 1.0 which is putrid for a center whose 5V5 P/60 ranged between 1.67 and 2.83 P/60 between these regular seasons and that’s not something you can place any sort of blame on Nabokov or Niemi for. 360 minutes in different series against different opponents with different linemates is also plenty enough of a sample size for me to say Thornton, the best player, regular season career wise, in every single matchup besides Crosby, is the primary reason for his own 5V5 GF/60 being so hideously low.
Thornton played 13 playoff series during those seasons and lost 8 of his matchups in GF%, 7 of those 8 in one of CF%, FF%, SF%, or SCF%, and 4 of those 8 in 3 of CF%, FF%, SF%, and SCF%.
I can promise you that if you pull up Toews, Kopitar, and Bergeron’s head-to-head matchup numbers in the playoffs, you will find something very different; specifically in the goals for column.
With some added context in certain areas, perhaps these numbers don’t all look as disgusting as they currently do. Perhaps But, for a #1C who consistently dominated CF%, FF%, SF%, SCF%, and GF% in the regular season, and scored a ton of points, his playoff CF%, FF%, SF%, SCF%, and GF% in these head to head matchups, along with his production, is just not up to par whatsoever. And it’s not at #1C level.