DENVER — Amid the excitement over the Cubs’ calling up the top prospect in their system on Monday was also the bad news that came about the team’s closer.

As they battle for a playoff spot in the closing weeks of the season, they’ll be without Adbert Alzolay.

The pitcher is headed to the 15-day injured list as he deals with a right forearm strain, retroactive to September 10. Alzolay last pitched on Saturday against the Diamondbacks, pitching a shutout inning despite two walks and a hit in a 3-2 extra-inning loss to Arizona.

First a starter for the Cubs and now a reliever, Alzolay has taken over the closer’s role for the club over the last few months. He’s converted 22 of 25 save attempts this season while sporting a 2.71 ERA with seven holds.

Alzolay has had two of his three blown saves since August 30 and didn’t pitch from September 2-7.

Reliever Michael Fulmer is coming off the injured list as the closer goes on it, joining the big league club along with Armstrong. The No. 1 rated prospect in the Cubs’ system is appearing at the major league level for the first time, two years and two months after the team acquired him from the Mets in the Javier Baez trade.

Another prospect who recently got called up, Alexander Canario, is going back to Triple-A Iow while pitcher Anthony Kay was designated for assignement.

The Cubs begin a three-game series with the Rockies on Monday evening at Coors Field.