Left off the roster was right-handed reliever Carl Edwards, who has pain in his forearm, according to the team.
Strop, 33, has been out since Sept. 13, when he injured himself running out a ground ball to first base. He has been the Cubs' de facto closer since the All-Star break, when Brandon Morrow went down with an arm injury.
Initially, Strop was pointing toward the beginning of the division series on Thursday for his return, but it was pushed up after the Cubs lost the NL Central tiebreaker on Monday.
Strop first told ESPN's Marly Rivera on Monday that he would be ready for Tuesday's game. Cubs manager Joe Maddon said on WSCR radio Tuesday that he would use Strop "almost like normal."
The Cubs have enacted Plan B in several places, including having starters Cole Hamels and Kyle Hendricks on their wild-card roster. After using six relievers in Monday's 3-1 loss to Milwaukee, Maddon indicated Hamels could be used out of the bullpen Tuesday behind starter Jon Lester. Hendricks is likely there for emergency purposes only. If the Cubs beat the Rockies, Hendricks will start Game 1 of the NLDS against the Brewers.
The Cubs kept 14 position players for the wild-card game, including speedster Terrance Gore.
Strop has a 2.26 ERA, 13 saves and a 0.99 WHIP this season.
A-Rod: Lester was 'made for' NL wild-card game
Alex Rodriguez explains that both the Cubs and the Rockies are overcoming fatigue and predicts that it'll be the Cubs advancing on Tuesday.