I get the concern, and I get that it's September and we're bored and feel like being concerned. And I understand that Dooling's retirement was unplanned and surprising, but all that said, I'm not worried. NBA teams are limited to 15 players on the roster. Essentially that means you have 3-4 players per position, since some players can handle multiple positions. If we say that the roster when at full strength, features Rondo, Terry, and Bradley as the PG options, that's a pretty solid roster. Of course, we're down Bradley for 20-25% of the season, so that creates the risk that if we're without JET or Rondo in that time, we're down to one PG option and thus desperate. It's a risk. But at some point during the season, most teams are down multiple players at the same position. You hope it's a short period, because if it's a long period you're probably in trouble, but it's life with a small roster and a very limited pool of viable replacements. Every team deals with it.
That said, to play along I'll assume JET goes down for a couple weeks while Bradley is still out (because Rondo is such an integral component of the team that replacing him is a different story, and it wouldn't really matter if both JET and AB were healthy or not.) I'd firstly hope that Christmas gets a little run at the lead guard position, to see what we've got there. His contract is half-guaranteed for the season, which means that we can cut him and sign a vet after the mid-way point without any cap hit. So in the event we go down another guard early, Doc may be forced to give him some run, and it may actually be for the long-term good.
Otherwise, Pierce can play his point forward position for a few minutes a game, and Rondo probably has to log a few extra minutes. Sub-optimal, obviously, but life.
If Lee goes down instead of JET, then we get to see if Christmas can play the 2 (which is a much better position for him) and we can again see Pierce playing some there as well. It's a very versatile roster, and can handle losing two players at the same position better than many teams (my biggest concern is losing Pierce and Green simultaneously, because that will lead to some weird Lee/Joseph/Christmas/Bass mashup), but losing two players who fill similar roles will always stress any roster. (See: our center position the last 4 years.)