The Chicago Cubs have added a much-needed power lefty arm to their bullpen in Aroldis Chapman.

Chicago sent four players to the New York Yankees in exchange for Chapman.

It’s the third trade between the Yankees and Cubs since July of 2013. Chicago sent Alfonso Soriano to the Yankees for minor leaguer Corey Black in July that year. They also traded Starlin Castro for Adam Warren and Brendan Ryan in December of 2015.

The Cubs’ bullpen looked good in April but has seen its production drop every month since then. Chicago relievers posted a 2.72 ERA with one blown save in April, a 3.80 ERA with two blown saves in May and a 3.96 ERA with four blown saves in June. In July, Chicago’s bullpen has a 4.64 ERA (24th in Major League Baseball) and three blown saves (tied for fourth most).

Chicago ranks 14th in the majors in bullpen ERA on the season at 3.83.

Cubs left-handed relief pitchers have allowed a .956 OPS in July. That’s the sixth-highest OPS allowed by left-handed relief pitchers this month.

Chapman’s opponents have a .351 OPS in July, fourth lowest in MLB.

Elite fastball

Chapman has been the hardest-throwing pitcher in baseball since entering the league in 2010. Over the past seven seasons, Chapman has thrown 1,513 pitches at 100.0 mph or faster, more than the next 18 pitchers on the list combined. His career average fastball velocity is 98.7 mph, highest in the majors.

But Chapman’s average fastball velocity for the Yankees this season has been well above his career average. It’s up to 100.0 mph, 2 mph faster than the next-closest player, Noah Syndergaard (minimum 200 fastballs thrown).

Forty-four of Chapman’s fastballs have reached 103 mph this season, already 16 more than his previous career high for a season. Only one other player in the majors has thrown a pitch that fast this season, the Braves’ Mauricio Cabrera, who has thrown four.

Elite results

The high-powered fastball has led to the highest strikeouts per nine innings rate in MLB history and very few hits.

Chapman joins the Cubs with a career rate of 15.2 strikeouts per nine innings pitched, 0.7 better than Craig Kimbrel in second place (minimum 250 innings pitched).

No pitcher has allowed a lower opponent batting average on fastballs over the past seven seasons than Chapman's .171 (minimum 1,000 plate appearances ending with fastballs).

He has allowed a .157 opponent batting average on all of his pitches over the past seven seasons, lowest out of 533 qualified pitchers. Chapman’s opponent well-hit average of 9.8 percent ranks second in MLB on that list behind former teammate Dellin Betances (9.5 percent).

Chapman has at least 30 saves and 100 strikeouts in each of the previous four seasons. That's the most such seasons in MLB history.

NL Central context

The St. Louis Cardinals have not scored a run against Chapman since 2011. Chapman has had 25 straight scoreless appearances against the Cardinals with 46 strikeouts since last allowing a run to them.

Chicago will be happy to have Chapman on their side. Since his first appearance against the Cubs in 2011, Chapman has allowed six earned runs in 37 innings against Chicago (1.46 ERA) with 65 strikeouts.