Datome has been outplaying him. If he wants minutes, he has to make shots and play better, especially in practice.
In hindsight, looking at when Datome got here in a trade, I suppose we shouldn't as surprised this is happening. Stevens has been dying for some veterans on this team, guys with some professional basketball tread. And while Datome hasn't been in the NBA very long, let's not forget that he was the MVP of a few different Italian leagues, so he has skills and has proven more than James Young.
But I agree with everyone else, he's only 19...he's got a long way to go. He'll get his shot eventually, even if it's not with the Celtics