Spence, 24, is a left-hander with two years of Major League experience. He started his career with an 0-2 record and 2.73 ERA in 40 appearances for the Padres in 2011.
This past season, he appeared in just 11 games with the Padres, posting a 4.35 ERA in 10.1 innings. However, he was 4-2 with a 4.20 ERA in four starts and 27 relief appearances with San Diego's Triple-A affiliate.
Herndon, 27, underwent Tommy John surgery in June after posting a 4.70 ERA in five appearances with the Philadelphia Phillies. In three seasons with the Phillies, the 6'5" right-hander posted a 2-8 record with a 3.85 ERA in 117 innings pitched.
Herndon was claimed by the Blue Jays in late October, then designated for assignment once again eight days later.
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