It's a slight downgrade. Mason is a better receiver than Cotchery, while Braylon is better than Plax. I thought Tanny did a good job knowing we could have only kept 1 of the 2 WR, and with this crap Cotchery pulled about wanting to be released. Even as a slight downgrade, its still a well above average receiving core. On top of that, this isn't a pass first time to begin with. Even with those 3 receivers last year we were still ranked in the lower third of the league in pass offense. I really don't see this as a big problem.
Now if we didn't get Mason, I would have been worried, since teams would've just doubled Holmes, making Sanchez rely on a receiver who hasn't played in 2 years.
The Jets are essentially fielding a very similar team from last year minus our receiving corps.