Robertson was optioned to Triple-A Oklahoma City on Friday, Mike DiGiovanna of the Los Angeles Times reports.
The right-hander was sent to Triple-A to make room for Daniel Hudson (knee), who was activated off the 60-day injured list. Robertson showed some decent flashes during his short time in the Los Angeles bullpen, but a 9.39 ERA and 2.22 WHIP in 7.2 innings suggests there's more work to be done before he'll be a reliable option.