After a promising start to his second NBA season, everything changed for MarShon Brooks when he stepped on Andray Blatche’s foot during a shootaround in Orlando before the Nets’ fourth game of the season. Brooks averaged 10.3 points in about 20 minutes through three games, but after sitting out three contests with an ankle injury, he never found a consistent place in the team’s rotation. “It was tough,” Brooks said yesterday after being introduced, along with Keith Bogans and Kris Humphries, as the newest members of the Celtics as the counterbalance to Paul Pierce, Kevin Garnett and Jason Terry going to Brooklyn. “The reason why it was tough was because I never really felt that I got beat out of my situation. I never really feel like I got beat. “I feel like it was given ... my minutes were given away. I was playing the first three games of the year, I was playing 25 minutes and shooting 60-something percent from the field at the time, went down with a freak injury in shootaround, and when I came back, nothing was the same. “So I felt like honestly I was on my way to a really good year if things had stayed that way ... but I learned a lot, man. I learned a lot.”Things turned out differently, as Brooks spent much of the year sitting behind more experienced players with the playoff-bound Nets. Now, though, he’s excited for the possibilities in Boston, and the potential of playing alongside All-Star point guard Rajon Rondo.