I think it's more that he only just recently became expendable. Until mid/late last season, we really only had Kadri and Bozak at C, with Nylander maturing in the AHL. Nyander didn't make the team until right after the NHL trade deadline, so there was no time to trade Bozak, and he wasn't really that expendable anyways - Kadri/Bozak/Nylander made 3 decent top 3 centres, Laich was a stretch in that role.
Nylander should be clearly better than Bozak by training camp. Kadri already is, and with us winning the draft lottery, I think Matthews will be above him right away as well. So that makes 3 centres above Bozak, or up to 4 if we sign Stamkos. So he's truly expendable now, but he definitely was not before last year's trade deadline. I'm pretty confident we deal him this offseason.
