It speaks to the mental poise of 19-year-old J.T. Miller that he has not been flustered by the fluctuations of the Rangers’ roster as he settles into his NHL career. The most shocking moment should have come on Thursday, when fellow rookie Chris Kreider was sent down to the Connecticut Whale of the AHL for his second stint in the minors since the regular season started. “I don’t really worry about that stuff,” Miller said before last night’s 3-2 shootout win over the Sabres at the Garden, his 12th game in the league since being called up. “I’m just focused every day on being here and what I have to do to get better.” Miller started the game on a line centered by Brian Boyle, with returning leading goal scorer Marian Gaborik on the other wing. He finished it with Taylor Pyatt in place of Gaborik, and was assertive in getting a team-high nine scoring chances: three shots, two shots blocked and four shots missed, all in 13:42. “I’m getting more comfortable,” Miller said afterward. “I just don’t want to skip any steps of the process. Instead of being relaxed and settled in, I just want to try and get better every game.” Miller still hasn’t scored since his two-goal outburst on Feb. 7, his first game at the Garden. He then went five games without a point, but had two assists in the past four games before last night. “It just shows nothing is permanent,” Miller said of Kreider’s demotion. “But it’s not something I’m concerned with.”