He is valuable to this franchise. I would tender the 2.1 or even see if he would take a multi-year deal, that would count less against the cap this year.
A couple of things to keep in mind:
He is on the same page with Dak. Before Dak went down, he was on pace for a 800 yard year.
He is a core special-teamer, he is not just a return guy. You don't find that very often out of a WR.
He backs up all three WR spots. If any of the big three go down, you still have a decent to good receiver to put in. That's very valuable for a team whose best chance for a playoff birth is to outscore their opponents.