Right-hander Anthony Varvaro, added on Tuesday after he was claimed off waivers from the Red Sox, was designated for assignment to make room for Grimm, 26, who opened the season with the big league team but never pitched, and was placed on the disabled list April 10 with right forearm inflammation.
The Cubs had been outscored, 20-8, in the sixth inning of games this season entering Wednesday's matchup with the Cardinals.
Grimm was pronounced ready after he made one rehab appearance, with Triple-A Iowa on Monday, throwing a scoreless inning with a walk and two strikeouts.
"I felt good about where I was at, [the ball] was coming out really well," Grimm said of Monday's outing. "The main thing for me is staying within myself now. I was really amped up and excited to be competing again."
He first felt discomfort in his right forearm in Spring Training.
"I wasn't able to throw a breaking ball [in spring]," Grimm said. "I didn't think I could give everything I've got to the team without a breaking ball. I'm ready to put it behind me and move forward."
Grimm didn't say much in Spring Training, because he didn't think the discomfort was that serious. But when he arrived in Chicago for the start of the season, his arm got "a little achy," he said.
"It was April, and time to say something," Grimm said.
He kept track of the Cubs while in Arizona on rehab.
"I watched nearly every game on my iPad in Arizona, and that wasn't fun for me," he said. "In another aspect, it was fun to watch this team. It was awesome to watch some of those games, the late-inning wins. I have confidence in everybody in the bullpen."
Last year, in his first full season in the Major Leagues, Grimm compiled a 3.78 ERA in 73 relief appearances.
Varvaro, 30, was claimed off waivers on Sunday, was added to the 25-man roster Tuesday and did not appear in a game. He was traded to the Red Sox last Dec. 17 and went 0-1 with a 4.09 ERA (five earned runs/11 innings pitched) in nine relief appearances this season before being designated for assignment on April 29.
Grimm joins lefty James Russell, who was called up from Iowa on Tuesday, as additions to the bullpen this week. Neil Ramirez, on the DL with a strained right shoulder, threw off the mound for the first time Wednesday and was making progress. He also was a key part of the 'pen last year.