Johnson was optioned to Triple-A Iowa on Friday, Gordon Wittenmyer of the Chicago Sun-Times reports.
In an attempt to keep him healthy, Johnson was shifted to the bullpen full-time for the 2017 season after injuries marred his 2016 campaign. He struggled to a 6.14 ERA in 63 Triple-A innings last season, so the Cubs will likely want to see Johnson perform well there before calling him up. If he's able to keep up his 10.7 K/9 and drop his 6.1 BB/9 at Triple-A, he could find himself in the majors sometime in 2017.