Belichick was crestfallen for Mayo because he's exactly what Belichick looks for in a linebacker. Mayo leads the team behind the scenes, calls the plays on the field and will be difficult to replace. The Patriots are thin at the position after losing Brandon Spikes in the offseason. Dont'a Hightower is currently hurt but could play more snaps at inside linebacker. The team gave a lot of snaps to undrafted rookie Deontae Skinner on Sunday in Buffalo. They could also use safety Tavon Wilson at times in more of a linebacker role on passing downs.
The Patriots had to replace Mayo last year in October when he tore his pectoral muscle and was lost for the season, so they have some practice. But it's a big loss for a defense that ranks eleventh in points allowed and fifth in yards allowed. The NFL season is largely about adjustments, and the Patriots have to make a few big ones this week.