You are correct. However, Salas was signed the day after the cutdown date and then waived 2 weeks later. By Sept 18, we were down to 3 + Slater.
A valid point has been made by many posters. Why do we need 5 wide receivers? Is it our norm to carry so many, especially given that we will carry 3 TE's, perhaps 4? We could start the season with 10 WR/TE on the 53 plus one or two on PUP (Gronk and Harrison). That is a lot!
After all, we had one of the top passing teams in NFL history last year. The total number of catches for the #3 and #4 receivers was 37, 38 if you include Stallworth's one catch. We ran a lot of 3 WR sets when one of TE's was out, and still the backup WR's didn't contribute much. Is it too much of a stretch to think that this year's #3 and #4 can produce as well as last year's and total at least 37 receptions. I think that the clear answer is "yes." In fact, it seems likely that the top 4 receivers may equal the production of the top 4 last year.
So, why are even considering a 5th WR? or a 5th RB for that matter? We are presuming that Belichick has DECIDED to use 2 positions on returners after using none last year. After all, Edelman was expected to be part of the offense, as he was when he was available.
Just maybe, we are correct and Belichick has made that decision. Perhaps, he has not.
Personally, I would carry the 5th WR. For me, it is just too much to depend on three rookies, and have no WR carried over from last year.
They started the season with 4 + Slater (Welker, Edelman, Salas, Lloyd).
Right now, I'd project 5 + Slater (Amendola, Edelman, Dobson, Boyce, Thompkins).