My two cents:
The rookie will play decent defense in a year or so and make Tony Allen a distant (although fond) memory. Tony was a nice defensive player on the perimeter but certainly not irreplaceable. Include Marquis Daniels and a healthy focused Delonte West and Tony is easily replaced, actually upgraded. Neither will put up the turnover numbers or bad shots like Tony has in his Celtic career.
With the addition of Shaq and JO, we already will have a much better second unit interior defensive presence in the lane. Their added length and size will take pressure off the perimeter IMO because we wont worry as much about Rasheed Wallace standing outside shooting 3's instead of rebounding or shot blocking.
I liked Tony, but he was just one guy on a team. His athleticism was not what I would call off the charts the past two seasons. I would attribute much his defensive prowess those two seasons as due to the 'system' and team. He will regret going to Memphis and not being in the post season. I bet he doesnt get more than 20 mpg as a Grizzly this season.
If our defense suffers this season it will be because of losing Tom Thibadeau or an injury to Rondo/KG, not from missing Tony Allen's contributions.