There's been no official announcement on this, but according to the Cubs' transactions page as well as the Pacific Coast League transactions page, the Cubs have optioned lefthander Gerardo Concepcion back to the Triple-A Iowa Cubs.
Concepcion made three appearances for the Cubs covering 2⅓ innings. He allowed two hits and a walk, one run and struck out two. It's too small of a sample size to draw any conclusions on his future, but the Cubs apparently feel he's not the answer to their bullpen issues, at least not right now.
Since the Cubs have not yet made an official announcement, we don't know who's going to replace Concepcion on the 25-man roster. Here's one idea, from a Des Moines writer:
That's possible, although Peralta is a righthander and the Cubs could use a lefthander to replace Concepcion. The 40-man roster currently stands at 39, so Peralta could be added.
We'll undoubtedly know more before tonight's game. Stay tuned.
UPDATE: By press release, the Cubs announced the optioning of Concepcion and addition of Peralta to the 25-man and 40-man rosters. Peralta will be available tonight, and wear #62. The 40-man roster is now full.