LHP Mike Montgomery
The Cubs front office struck gold when they traded for lefty Mike Montgomery in 2016. The 27-year-old was relatively unproven at the time, but he almost seamlessly stepped into a swingman/long-reliever role with great success. Montgomery made 17 appearances for the Cubs in 2016, with five of them being starts, with a 2.82 ERA in 38 1/3 innings.
His claim to fame to this day is the fact that he induced the final out in Game 7 of the World Series, but his production in the Windy City goes far beyond that. The swingman navigated in and out of the bullpen for parts of the next three seasons for Chicago, with a career 3.74 ERA with the Cubs. The swingman job is already a difficult assignment for any pitcher to handle, but the fact that Montgomery did it as a lefty and was still able to have success against hitters on both sides of the plate speaks volumes.
Montgomery only cost the Cubs first baseman Daniel Vogelbach (their 20th-ranked prospect at the time) and LHP Paul Blackburn, two guys who were not close to helping the big-league roster in 2016. This is definitely one of the more under-the-radar moves of the Theo Epstein era that helped sustain the team's rotation when injuries and poor outings popped up.
RHP Wade Davis
Former closer Wade Davis only spent one season in a Cubs uniform, but he was a lockdown presence in the back of the bullpen, contributing to a 92-win season in 2017. The Cubs acquired the stoic right-hander in the offseason after the World Series championship when the team did not pursue re-signing closer Aroldis Chapman.
Davis was acquired when the team sent Jorge Soler to the Kansas City Royals for Davis. At the time, Soler was a young slugger with a high ceiling, but he hadn't proved stable enough to avoid injury or produce at the plate at a level to warrant anything larger than a bench role for the Cubs. It was a solid move because Davis had a fantastic season.
The closer had a 2.30 ERA and 79 strikeouts over 58 2/3 innings pitched, and was nearly perfect in his 33 save opportunities with just one blown save. It was the type of season the Cubs would kill to see a relief pitcher put up today since the team is starved for a legitimate closer. It also capped off an incredible four-season stretch where Davis was arguably the most dominant pitcher in baseball, with three straight years of sub-2.00 ERA baseball.