Celtics coach Doc Rivers witnessed two of the flat out dumbest moves in Knicks playoff history — J.R. Smith’s suspension-arning elbow Friday and John Starks’ infamous 1993 head-butt on Reggie Miller — from both sides and 20 years apart.

Rivers was Starks’ teammate against Indiana in 1993 when after complaining about some elbows from Miller, Starks unloaded his head-butt. Rivers, as Celtics coach, saw Smith fling his elbow at Jason Terry, resulting in his suspension for yesterday’s 97-90 overtime loss. Both occurred in Game 3 of a first-round series and resulted in ejections. Starks was fined $5,000. Smith drew the harsher penalty with a one-game suspension.

Rivers knew Starks was on the verge of eruption back then.

“I had just gone to Jeff [assistant Van Gundy] and said, ‘You better get him out,’” Rivers recalled. “You could tell John was on the edge. The thing about John was he was a great teammate and a great guy off the court. On the court, he was the Tasmanian Devil.

“Something happened and I knew he was about to go off and I told Jeff to tell Riles [head coach Pat Riley] to get him out. Well, we all were a bit late and the next thing, bam,” Rivers said.