To clear roster space before Tuesday night's game against the Los Angeles Dodgers, the Royals optioned reliever Tim Collins to Triple-A Omaha and designated fellow left-hander Donnie Joseph for assignment.
Chen began the season in the Kansas City rotation before landing on the DL with a bulging disk in his back. He made four minor league rehab starts, three at Omaha, and struck out 11 over 5 1-3 innings last weekend against Albuquerque.
The veteran left-hander, who has worked out of the bullpen in the past, has not appeared in a big league game since April 24. He is 1-2 with a 7.45 ERA on the season.