The Celtics, who were paced by Paul Pierce's 18 points, led by six at half-time but were outscored 45-23 thereafter.
JR Smith scored 19 points for the Knicks and Raymond Felton added 16.
Dwayne Wade outshone LeBron James with 21 points as the Miami Heat beat the Milwaukee Bucks 98-86 to stretch their series lead to 2-0.
James added 19 points, while Ersan Ilyasova led the Bucks with 21.
Stephen Curry had 30 points and 13 assists as the Golden State Warriors eased to 131-117 win over the Denver Nuggets to level their series at 1-1.
Curry was one of four Golden State players to score more than 20 points, with Jarrett Jack pouring in 26, Harrison Barnes 24 and Clay Thompson 21.