An offer sheet of 7 years 44 million (~6.28 AAV) would still fall under the threshold of two first round picks, a second, and a third. An offer sheet of 7 years 35 million would fall under the one first, two seconds, and three thirds.
For deals over five years, they take the entire salary and divide it over just 5 years. With the minimum for 4 first round picks being about 9.1 million, on a seven year deal anything less than 45.5 million (5x9.1) would only cost a 2 1st, 1 2nd, and 1 3rd. Anything under 7.3 million would only cost Boston 1 1st, 1 2nd, and 1 3rd.
So it's not out of the possibility for an offer sheet to be worth it for Boston. 7 years at about 35 million means they only have to give up 1 1st, 1 2nd, and 1 3rd.