That would require cap space.
Celtics are hanging out right below the luxury tax level. If they feel like they're close going to the finals, especially without Lebron, they could still add a non taxable MLE player AND still be just under the level, which allows them to make a trade. I don't think their ownership is opposed to paying a tax when they're close to the finals based on economics, I think Danny Ainge just wants flexibility for trades because of their logjam of draft picks and guards.