Jax was optioned to Triple-A St. Paul on Wednesday, Brandon Warne of Access Twins reports.
Jax made his first career start against the Royals on Saturday and allowed six runs on eight hits and three walks while striking out three in five innings to take the loss. While the right-hander could be an option to return to the Twins as a starter or reliever at some point, he'll head to the minors ahead of the All-Star break after right-hander Michael Pineda (forearm) was reinstated from the 10-day injured list ahead of Wednesday's start against the White Sox.