Think about it. Avery Bradley and Courtney Lee will start together. JET and Barbosa will back them up. If the Celtics had to lose a player at any position, thankfully it was at the GUARD. Suddenly having all of these capable backup guards seems like it will pay off, as these backups will all see more defined roles.
With Bradley starting and playing well with Lee (especially on defense), and JET plus Barbosa getting more regular backup minutes, these guys will still be able to make it happen. Plus, the offense seems less "congested" without Rondo on the floor (at least with all of these new pieces). Don't get the wrong idea, I love Rondo. To contrast, if it was KG that went down, we would be unable to make the playoffs.
Rondo (though he played exceptionally well in the games prior to the injury, and was scoring late game points) is NOT the go-to guy at the end of games. The Celtics have other capable passers in Bradley, Pierce, Lee, and even Terry/KG. Pierce will always take the game-winner regardless of Rondo's status.
If this team gets into the playoffs, or loses in any round, it won't be for any reasons involving Rondo not being around (e.g. rebounds, points in paint, etc.)
It's not the end, just a new chapter!