Drew Smyly and the 60-day DL

2/16 UPDATE: 

The Cubs have placed Drew Smyly on the 60-day DL to make room for RHRP Shae Simmons on the Cubs MLB 40-man roster. 

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MLB clubs can place an injured or ill player or a player rehabbing from an injury or an illness on the MLB Emergency Disabled List (AKA "60-day DL") starting with the first day of Spring Training (the day pitchers & catchers report), which in the case of the Cubs is this coming Monday (2/12). 

THE 60-DAY DL: 

A club cannot place a player on the 60-day DL unless and until the club's MLB Reserve List (AKA "40-man roster") is full, although in practice clubs don't place players on the 60-day DL (or transfer a player from the 10-day DL to the 60-day DL) unless the 40-man roster is full AND the player's slot on the 40 is needed for another player. Right now the Cubs MLB Reserve List has 39 players on it (one below the limit), so no Cubs player can be placed on the 60-day DL until one more player is added to the 40. A player who is placed on the 60-day DL during Spring Training must remain on the 60-day DL for at least the first 60 days of the MLB regular season, so time spent on the 60-day DL during Spring Training does not count toward the 60 days.   
NOTE: As long as the player did not appear in a game (including "official" MLB Cactus League or Grapefruit League games or other MLB pre-season exhibition games) during the retroactive period, an MLB 10-day DL assignment can be backdated up to three days (including the three days prior to MLB Opening Day), and a 7-day (acute concussion) DL assignment can be backdated up to four days (including the four days prior to MLB Opening Day), and (unlike the 60-day DL) time spent on the MLB 10-day DL or 7-day (acute concussion) DL during Spring Training does count toward the minimum number of days a player must spend on the DL. 

MINOR LEAGUE REHAB ASSIGNMENTS:

In most cases a pitcher can spend no more than 30 days and a position player can spend no more than 20 days on a minor league rehab assignment, but beginning with the 2017 season (and with the approval of the MLB Commissioner) a pitcher or a position player who has undergone elbow UCL reconstruction surgery (so-called "Tommy John Surgery" or "TJS") may have his Minor League Rehab Assignment renewed for up to 30 additional days (in the form of three ten-day renewals). Since time spent at Extended Spring Training is not considered a minor league rehab assignment, a player rehabbing from TJS can spend April-June at Extended Spring Training, and then begin his "official" Minor League Rehab Assignment on July 3rd, with the 60 rehab days expiring on September 1st when MLB Active List limits expand from 25 to 40 players.  
NOTE: Extending a Minor League Rehab Assignment an extra 60 days ONLY applies to pitchers or position players who are rehabbing from TJS. A Minor League Rehab Assignment for a player rehabbing from another type of injury or illness (even severe injuries or illnesses that might require a long rehab period) are not eligible to have their Minor League Rehab Assignmment extended.  

DREW SMYLY: 

The Cubs signed LHP Drew Smyly to a 2018 major league contract (and added him to their MLB 40-man roster) in December after he was non-tendered by the Seattle Mariners. Smyly underwent elbow UCL reconstruction surgery (TJS) last July and almost certainly won't be ready to pitch in games until at least mid-season 2018. So look for Smyly to spend three months at Extended Spring Training (April-June) at the UAPC in Mesa working at his own pace, and then begin a Minor League Rehab Assignment no earlier than July 3rd, probably starting out at one of the Cubs two AZL affiliates and then gradually moving up to higher levels over the course of the 60 days until the rehab has been completed and he is deemed ready to pitch in MLB games. Then he could be reinstated from the 60-day DL on September 1st (when MLB active list rosters expand), presuming no medical setbacks occur along the way.