If they don't get an expiring contract for Ray Allen, then their payroll for next season would be extremely high. They're already at $65M, adding $18 millions or so would put it at $83 millions for 10 players or so. Not happening. With the lux. tax. threshold expected to come down to around $64M, the Celtics would be risking to pay above $110 millions in salaries for a team whose chances for contending would be unknown.
The Celtics are a highly leveraged franchise, I doubt they'll trot out next season with a payroll similar to the one of the last 2 seasons. If Ray Allen is traded, we'll get some expirings back in order to make up for Rondo's raise next season.