The Cubs have reinstated 3B Ian Stewart from the 15-day DL and have optioned him to AAA Iowa. (Stewart came into the 2013 season with one minor league option year left).
Stewart suffered a strained quad in an intrasquad game at HoHoKam Park in February, and missed the entire 2013 MLB Cactus League Spring Training schedule. He was placed on the Cubs MLB 15-day Disabled List on March 31st (retro to 3/22), and was left behind at Extended Spring Training when the Cubs left Arizona at the end of March.
Stewart hit 200/250/267 in 16 PA at Extended Spring Training, and then was moved up to AAA Iowa on a minor league "rehab" assignment on April 14th, where he has hit an ice-cold 091/255/114 in 58 PA.
Since the maximum number of days a position player can spend on a minor league rehab assignment is 20 (it's a maximum of 30 days for pitchers), May 3rd is the end of the assignment (time that was spent at Extended Spring Training prior to going to Iowa is not considered part of the minor league rehab assignment). However, a club does not have to reinstate a player to an active roster after the conclusion of a minor league rehab assignment if the player is not healthy enough to play (like if the original injury or illness reappears and/or was aggravated or if a different injury occurred during the course of the rehab assignment).
Because Stewart made his debut on an MLB 25-man roster more than three years ago, Optional Assignment Waivers had to be secured before he could be optioned to the minors. Optional Assignment Waivers are revocable (meaning the request can be withdrawn if the player is claimed), and once secured they are good for the entire waiver period. So Stewart can be sent back & forth to the minors without any restrictions (that is, he can be recalled and then be optioned to the minors again anytime during the current waiver period without Optional Assignment Waivers needing to be requested & secured each time). The current waiver period just started this past Tuesday, and it doesn't end until 4 PM (EDT) on July 31st.
BTW, although Optional Assignment Waivers had to be secured before Stewart could be optioned to the minors, Stewart did NOT have to give his consent to the assignment. Only players who have accrued at least five years of MLB Service Time must give their consent before they can be optioned to the minors, and Stewart (if he had remained on the 15-day DL or if he had been reinstated to the 25-man roster instead of being optioned) would not have reached five years of MLB Service Time until July 31st. (And now that he's been optioned to the minors, Stewart reaching five years of MLB Service Time will be even further delayed).