There is nothing negative about Lange's stats right now. I just don't have enough information about him to project his future results with any accuracy. most pitchers with low BABIP and 0 home run rate don't sustain that performance. We already know Hader is one of the relatively rare pitchers who can keep his BABIP consistently low, so that is not a factor for him. Because he can keep his hit rate so low, his high above average walk rate is also not a factor.
I am not really concerned about Lange. I don't really have a strong opinion on him. He's a failed starter having 40+ innings of success as a reliever. That is a positive, but not enough information to say that he'll be a top reliever in the future.
I am not advocating trading Lange at all. I just don't think he is such a sure thing that I am going to make him untouchable. I don't know that he should be any less tradeable than Will Vest who has even better numbers this year. I wouldn't trade anyone for prospects. I am done with that, but if they can package some relivers for a hitter who can help the next couple of years, I'd be all for it.