The bruising forward was a much-needed addition, filling a gap left by an injured big man with energy, rebounding, and scoring. Ken­yon Martin provided life to a squad that was very much in need. But Martin didn’t do all that for the Celtics, for whom all the above stipulations apply. Martin instead signed with their rival to the South, the New York Knicks, joining them on a 10-day contract last month and then signing with them for the remainder of the season. “It’s their fault,” Martin said Tuesday night after scoring 9 points and grabbing five rebounds in the Knicks’ 100-85 win over the Celtics at TD Garden. “They lost. There was talks, there was negotiations, they chose not to do it. It was out of my control. I’m a Knick now, so they lost.” Martin has provided quite a spark for the Knicks and has helped make up for the absence of Amar’e Stoudemire, who was expected to miss six weeks with knee surgery. The 35-year-old, who is in his 13th season, entered Tuesday having averaged 14.3 points and 7.5 rebounds in 28 minutes in the Knicks’ previous four games, each of them wins. “I’m just here to prove to people that I’ve never lost it,” Martin said. “I guess I’m a better person than I am a basketball player. But the chip I’ve always played with hasn’t gotten bigger. I’m here to prove what I can do — that’s everybody, every night. “It doesn’t start with the Celtics. Whoever puts that uniform on is opposite us. “They’re going to see what they’re missing.”