Assuming the NBA grants the Cs a Disabled Player Exemption of around $3.5m (half the QO for RFA Green), they should be able to get one of AK-47, K Mart or JR Smith for that on a one year deal, considering that there's almost no money left out there on NBA rosters for contending teams.
I'll take any one of them. AK could be awesome here. Him, KG and Rondo on the court at the same time is foreboding for the other team's offense.
KMart, although I personally despise, plays with an edge that could be useful.
JR Smith is a knucklehead, but has top of the league talent, and with KG, RA, and PP as mentors and Rondo as the conductor, could be 6th Man of the Year here.
Time for Ainge to make lemonade from this situation.