I just don't see Boston matching a team offering BBD $3.5 plus or their MLE when it's obvious the C's need to spend their money elsewhere (veteran back-up PG and SF) so a team making that kind of offer would essentially be signing him "outright" (although they'd have to sweat out that 10 day waiting period the C's would have) because of the difficulty Boston would have in justifying matching it.
Boston could very likely match that kind of offer because spending money on BBD isn't exactly using money that they would otherwise offer to somebody else.
All we can offer to players besides BBD at this point is the LLE and the Vet. Minimum. We can offer BBD more without it counting as part of the MLE, so we likely would as long as it doesn't go too far over 3 mil.
