Jared Sullinger knows what it’s like when fortunes rise and fall during the NCAA tournament. The second-year Celtics forward went 5-for-19 from the floor on the way to 13 points in his last college game, an Ohio State loss to Kansas in the 2012 Final Four. “In the back of my phone,” Sullinger said yesterday, referencing a bit of motivation he’s retained, “Chad Ford was on (ESPN’s) ‘First Take’ and Stephen A. Smith said, ‘I don’t know if he’s a first-rounder yet. I might not even draft him.’ It kind of reminds myself why I’m out here, what I need to do. It kind of puts fuel to the fire, so I remember two years ago when everybody said I couldn’t play in the NBA. I was too small. I wouldn’t be able to score. All that stuff. I look at those guys and I kind of laugh, but I’ve still got to prove people wrong.” Sullinger ended up being selected 21st overall by the C’s and has done quite well in the league, especially as he works his way into a better groove after back surgery shortened his rookie season. He’s averaging 12.8 points and 8.2 rebounds in 65 games this season — and he still has room for improvement. “We started talking about that a while ago,” coach Brad Stevens said of the offseason blueprint for Sullinger. “He’s got a game plan of what he wants to do from a strength, conditioning and playing standpoint, and we’re all on board with that. We’ve already discussed that in pretty good detail with him. I think it’s most about conditioning. It’s mostly about being able to play longer stretches as effectively as possible.