Other moving parts in the deal include Jordan Pries, who heads to the Cubs. Dan Vogelbach and Paul Blackburn were also in the trade, both going to Seattle.
The 27-year-old Montgomery was a 1st round (36th overall) pick by the Kansas City Royals in 2008, but he did not break into the Major Leagues until he pitched with the Mariners last season. He was used exclusively as a starter in 2015, pitching to subpar results, posting a 4.60 ERA in 90 innings pitched.
This season, however, Montgomery has pitched a lot more out of the bullpen for Seattle and has found some success there. Like many flameout starters (Wade Davis), Montgomery has been very good out this year, seeing his strikeout rate tick up and his walk rate go down. This has led to better results.
In 32 games (two starts), Montgomery has posted a 2.34 ERA, a 3.20 FIP and a 3.54 xFIP. In all, he has been worth 0.9 fWAR and has generated more swinging strikes, while seeing his velocity increase across the board.
The addition of Montgomery is big for the Cubs, who have been in need of bullpen help. While he's not a flashy addition, Cubs' relievers have combined to post a 0.9 fWAR this season, 23rd in the Major Leagues. But they have a 3.82 ERA, right in the middle of the pack.
Montgomery will help out at the back-end of the bullpen in Chicago, where he has done so thus far in Seattle.
The Cubs also received Triple-A right-hander Jordan Pries in the trade. This season, Pries has a 3.65 ERA and a 21 to 7 strikeout-to-walk ratio in 24 2/3 innings pitched there, coming over seven appearances (three starts).
For the Mariners, the trade of Montgomery is an interesting one.
The team was expected to contend this season and did open up the year playing well, but they have found themselves in a tailspin over the past few weeks, falling in the American League West standings. At 48-47, the Mariners are seven games out of the division race and are five-and-a-half games out of the Wild Card.
While trading Montgomery does not represent a complete sell-off in Seattle, it's hard to expect them being major buyers at the trade deadline even still.
Vogelbach is an interesting addition for Seattle, though, and he could help out at the Major League level this year. The 23-year-old defensive liability has been hitting to the tune of a fantastic .318/.425/.548 line this season with 16 home runs and 64 RBI in 365 plate appearances this year. He could be a good designated hitter candidate in the American League.
As for Blackburn, 22, he was the Cubs' 1st round pick in the 2012 draft (56 overall). He has thrown 102 1/3 innings pitched this season and has a 3.17 ERA as well as a 72 to 26 strikeout-to-walk ratio in Double-A.