They're the second seed in the east. They are contending right now. I'm not saying they are legit favorites or anything. Obviously the Cavs, Warriors, and Spurs are probably 95% likely to win this year. But crazier things can happen. The Rockets weren't supposed to be in the Finals in 86. Everyone thought it would be the Lakers, but the Rockets still took the Celts to 6 games. What if Bird had gone down in game 1? Maybe the Rockets shock the world and win a championship in a year everyone thought it was going to the Celts or Lakers. All it takes is LeBron to turn an ankle and have Durant's injury flare up and the Celts could steal one this year. Not likely but not impossible.
In terms of being a more legitimate contender on the level of the Cavs, Warriors, and Spurs, I expect that to happen next year. Let's say we win 53 games and advance to the 2nd round or ECF this year. We are set up to without a doubt add 1 marquee player to that team (draft) and likely a 2nd (FA or trade). You add 2 marquee players to our current core, and you should be talking about a team winning 55-60 games next year, a 1 or 2 seed in the East, and a legit top tier contender.