Now i hope he never ever gets injured, but if he does I hope it's in the regular season and not the playoffs, that being said, we have almost no trade chips(Tony Allen and Brian Scalabrine expirings)), weak prospects(Lester Hudson, JR Gidddens, Injury prone Bill Walker). The only players we will be able to trade for is a player with a contract longer then this season with a team looking to drop salaries,(example: New York, Milwaukee), an unhappy player who destroys his trade value by publicly demanding a trade(I dont know who this would be, but maybe like Monta Ellis or something), or a player who is so bad and has so little value to his existing team that he wont be worth acquiring in the first place. To make things more complicated his salary must not be too big that it will destroy the Celtics cap flexibility in the future(so no to Monta Ellis), his game must also be suited for what the celtics are looking for in a backup pg(strong D, good 3 point shooter, good ball handler, versatile). Now after looking at these Criteria, I think the player that makes the most sense to the Celtics is Charlie Bell of the Milwaukee Bucks.
A trade of Brian Scalabrine for Charlie Bell straight up works out, and for a team that traded away Richard Jefferson for cap relief, there's a good chance that they would trade away Charlie Bell away for cap relief as well. Even though his contract is relatively long, his annual figures are low enough(under 4 million) that finding a team willing to take his contract will not be hard if they needed to trade him in the future.
He is also capable of playing multiple positions so that even when Rondo is fully healthy, he can fit in very well with the backup unit with Eddie House and Marquis Daniels. He is a better ball handler then House or Daniels and he is a solid 3 point shooter. He is also known for his solid defence capable of guarding the 1 and the 2.
I really think that they will take a long and hard(no homo) look at Charlie Bell, and he is the most likely player to be acquired near the deadline.