A little maybe. Pass routes, snaps, etc. are timing based, so practice is fine for that.
Of course in games you're not always going to have clean snaps, due to crowd noise, etc. and defenders will knock receivers off routes slightly, at the least. But players should know how to deal with that, they're not playing football for the first time. Rookies do need to adjust to the NFL game speed, but Dallas isn't going to have more than a few rookies on the field at any given time.
This isn't the 50's or 60's or 70's where players just went home after the season and worked other jobs or went fishing, they have conditioning programs to do, etc. Football's a year round job now.
Even if they are rusty, there's 17 games to play, all but Dallas and Pittsburgh only will play 3 preseason games, and most starters will play sparingly, if at all.
Being "rusty" isn't much of a concern for me...