Middleton made the big club during the 1974–75 season, and despite suffering injuries that restricted him to 47 games, scored 22 goals in that limited time.

Rangers head coach and general manager John Ferguson Sr. was confident that his team had enough young talent to justify making Middleton expendable.

[2] The transaction became even more one-sided in favor of the Bruins when head coach Don Cherry developed Middleton's defensive skills to make him a solid two-way player.

Generally paired with centre Barry Pederson, Middleton had five straight seasons of at least forty goals and ninety points and led the Bruins to perennial glittering records.

His leadership was apparent in being named co-captain (with Ray Bourque) to succeed Terry O'Reilly in 1985, a position he held until he retired, wearing the "C" during home games.

At the time of his retirement following the 1987-88 season, Middleton ranked third on the Boston Bruins' all-time regular-season scoring list (trailing only Johnny Bucyk and Phil Esposito) with 402 goals and 496 assists for 898 points--10 more than Bobby Orr accumulated.

[5][6] Joe McDonald speculated in a May 14, 2020 article in The Athletic that Middleton's participation in the lawsuit jeopardized his chances of induction into the Hockey Hall of Fame.