Howard was optioned to Triple-A Albuquerque on Monday, Patrick Saunders of The Denver Post reports.
The 25-year-old stood virtually no chance of cracking the Opening Day roster, as his presence on the 40-man roster can largely be chalked up to protection from the Rule 5 draft. Howard gave up seven earned runs in 12.1 innings this spring (5.11 ERA). He'll likely join the Isotopes' rotation after making his Triple-A debut last season and producing a 3.89 ERA in 14 starts.