Underwood was optioned to Double-A Tennessee on Monday, Patrick Mooney of CSN Chicago reports.
Underwood will head back to a level where he struggled at last year, posting a 4.91 ERA and missing significant time with an undisclosed injury before ending his year in High-A Myrtle Beach. The 22-year-old will look to work his way up through the Cubs' minor league ranks in 2017, although he remains a long way away from the majors.