One of the problems with threads like this is that fans think of last year's team, then subtract the guys who left -- what this thread is about -- then add in new guys about which something is known, typically veterans, then figure they can calculate what this year's team is going to be like.
But BB's take, which I think is likely to be more on the money, is that it's a whole new team. Some of the players were on last year's team. Some of the veterans will be older and slower, perhaps to a degree that makes them hardly useful at all, or at least in some cases will make them less useful. Some of the veterans are relatively young and will play a lot better. Some of the guys that are new will catch on and play a lot better than you might expect; some won't catch on at all and may not even make the team.
And then there's the rookies, the rookie free agents, and the guys you add on in training camp, all of whom are pretty much unknown.
It's a whole new team, with some elements you already know about, and how it comes together is pretty unknown, except for Pats Fans, who are able to count on it being pretty good.
By the way, Mike Gillislee is 218-219 lbs. I had remembered 220, so I looked it up. Not a big back, but big enough to make an impression on a tackler. A lot bigger than James White or Dion Lewis.