I think this article is a little behind the times. Duffey's three highest-leverage appearances have all been in the month of June. Two successes, one fail IMO (whether or not the runs counted as earned). 21 batters faced in those 3 games, 12 outs recorded. Jury's out, I'd say, but it's not been stellar. His OPS is better than league average, and for relievers I find that a simpler way to assess results than to pick through unearned runs and inherited runners and so forth. I guess the braintrust noticed too, and decided to start testing him a little more when it matters, about three weeks ago.