The move Sunday gave the Rangers another position player with first baseman Prince Fielder unavailable until at least Tuesday with a stiff neck.
Robertson made his big league debut with the Rangers on April 29, six days after he was acquired in a minor league trade with San Diego. He hit .182 in six games. Robertson was hitting .222 in seven games with Round Rock.
Mikolas didn't appear in a game for the Rangers after being recalled Wednesday. He had eight relief appearances with Round Rock after starting the season on the disabled list.