Looks as if Hertl will hit the 7-year rule for UFA one year before the age limit. If Hertl's knee becomes a non-issue, this one is likely to bite DW in the butt.
Hertl's the future 1st line center to replace Thornton, not Couture. Couture is very good, and probably the best 2-way forward on the team, but he doesn't have Hertl's size and physicality particularly on puck possession on behind the opponents net.
I've been near both in street clothes and he's does not appear to match the roster listed height and weight, but Hertl does appear so.
Of course, I was right next to 5 Blues players in street clothes a couple of seasons back, and all of them did not appear to match their roster height and weight. There's has to be some gamemanship in NHL rosters.