In 38 games this season, Smith is 1-4 with a 3.82 ERA and six saves.
The Cubs continue to stock up their bullpen as they prepare for a second consecutive playoff run. In the past month, they have acquired closer Aroldis Chapman and Mike Montgomery and activated Joe Nathan, whom they signed in May.
Those acquisitions have picked up a unit that saw Trevor Cahill hit the 15-day disabled list on July 15 with right patellar tendinitis and a struggling Justin Grimm get optioned to Triple-A.
Smith will bolster the bullpen further while bringing a different style of pitching to the mix.
With a unique delivery between a sidearm and submarine motion, the right-hander has fared better against left-handed batters this season, holding them to a .246 average compared to a .267 mark against righties.
The Cubs bullpen is already the best in the majors, allowing just 3.51 runs per game. The emergence of Carl Edwards, who has posted a 1.62 ERA in 15 appearances, has worked well alongside Pedro Strop, Travis Wood and former closer Hector Rondon.
Due to Chicago's embarrassment of riches in terms of late-inning arms, Smith could make appearances in the middle of games, which is different than his role with the Angels, as he was largely used as a setup man.
But with him going from a struggling American League team to a National League contender, Smith's depth will preserve Chicago's most important arms as the season wears on.
In return, the Angels will get a pitcher in Castillo who is honing his game in Single-A ball, holding a 2-3 record with a 3.27 ERA, per MiLB.com.
Stats courtesy of Baseball-Reference.com.