Let's compare the two pitchers then. Here are their numbers since Tazawa returned from TJS as a reliever.
Tazawa - 1.89 ERA, 2.63 xFIP, 1.02 WHIP, 0.94 BB/9, 9.79 K/9, 0.59 HR/9 IP
Papelbon - 2.30 ERA, 2.99 xFIP, 0.98 WHIP, 2.03 BB/9, 10.87 K/9, 1.01 HR/9
Am I missing something here? Tazawa appears to have performed better. So why is it that Papelbon would be worth a top 15 prospect and $14 million a year?