Rivera, 32, has a 0.53 ERA, 5.8 K/9, and 3.7 BB/9 in 17 Triple A innings so far this year. He's got similar peripherals over 275.6 career big league innings, leading to a 4.05 ERA. Rivera was released by the Nationals in December (on his birthday) to make room for Brian Bruney. He later signed a minor league deal with the Indians.
Howry, 36, was signed by the D'Backs to a one-year, $2.25MM deal in December. In 14.3 innings this year, he sports a matching 3.8 in terms of home runs, walks, and strikeouts per nine innings. That's not an effective formula. Arizona's bullpen has a 7.68 ERA in 104.3 innings, so changes were necessary. Aside from Aaron Heilman, everyone has struggled.