The word "bust" certainly won't be appropriate. If he reverts to expected form the rest of the year, he's still had an incredible season. If you ever start seeing "Roberts is a bum" posts, then the author will have an i.d. like "imachump" or "couldiBEmorestupid." If you drafted him or plucked him off the FA list, you struck gold, and there is no conceivable downside. If you sold the farm to trade for him when his value was off the scale...well, that's your bad.
Anyway, if you're interested in the fascinating world of opinions on Roberts, there is no shortage of lengthy posts in the Cafe on this subject. I'll give you a hint though as you're reading these -- nobody knows what he's going to do the rest of this year. And when you read fervent opinions on it either way, then the writer does not know what they're talking about. At the one-quarter pole, though, it's reasonable to think that his performance drops off considerably the rest of the season, but that it's still significantly better than his career numbers. If that's true, he's still an extremely value fantasy option.