The Chicago Cubs are undergoing a changing of the guard behind the plate. Willson Contreras, who has seven homers and a .780 OPS in his first 53 games, has seemingly earned a larger share of playing time.

Miguel Montero, who has caught Jake Arrieta regularly, has struggled at the plate, with five homers and a .197 batting average in 65 games.

Their numbers behind the plate are a slightly different story. With Montero catching this season, Cubs pitchers have an ERA of 3.20. That number is 4.01 when Contreras is catching.

The importance of called strikes

Over the past two seasons, Montero ranks among the best at getting calls for his pitchers. Adjusted strikes looking (SL+) measures how good a catcher is at earning called strikes compared to his peers, based on the likelihood that those strikes are called by an umpire.

According to that metric, Montero ranks seventh in the major leagues at getting calls for his pitchers (104.9) since the beginning of 2015.

Contreras has a 99.7 SL+ in 36 games catching this season, which is slightly above league average.

The first matchup didn't go so well

Last Thursday against the Brewers, Contreras caught Arrieta for the first time. Typically, Arrieta rates above league average at getting called strikes, thanks in part to his pairing with Montero.

In his first game with Contreras behind the plate, Arrieta had his second-worst game of the season at getting called strikes (88.1 SL+). While not necessarily a result of that, Arrieta also had a career-high seven walks, which is two more than any other Cubs pitcher in a game this season.

Along with it being their first game together, the duo also had umpire Rob Drake behind the plate calling pitches. This season, Drake ranks right at league average in terms of calling pitches as strikes. In his last few games, his SL+ has ranged from 85.5 to 122.3, so it's possible Contreras and Arrieta simply caught Drake on a day where his zone was small.

Against the Padres

Arrieta faces the San Diego Padres on Tuesday for the third time in his career and second as a member of the Cubs. The Padres enter Tuesday's game with a .238 batting average, lowest in the majors.

Against San Diego, Arrieta and Contreras might not need as many called strikes. The Padres swing at 47 percent of pitches, slightly above league average, and they miss on nearly 26 percent of their swings, the second-highest rate in the National League.