This is their third season playing together, and while their team might not be showing obvious signs of improvement, they are. Along with the dominance of Jeff Samardzija, that's been the biggest plus from the first six weeks of the season for the Cubs, who stayed sweep-free against their crosstown rivals on Thursday night.
Both Rizzo and Castro hit long two-run home runs to help the Cubs win, 12-5, at U.S. Cellular Field, saving a little civic face after dropping the first three games of the Interleague series. They are the No. 3-4 hitters in a lineup heavy on platoon players and newcomers, and they are not looking miscast, especially the last couple of weeks.
Manager Rick Renteria says Castro and Rizzo have "started to show some fruit" from their workouts in games.
"Now that you've got some games under you belt, you're starting to see some numbers -- some walks, some hits, some balls driven, things of that nature," Renteria said a few days ago at Wrigley Field. "As a consequence of things they've worked on, and now seeing some fruit from it, the confidence continues to grow. They're doing hopefully what everybody hopes they would do."
This is a very nice development for Theo Epstein and the Cubs' front office, which long ago bet heavily on these two 24-year-olds.
In their first major move, Epstein and general manager Jed Hoyer traded Andrew Cashner to the Padres to get Rizzo. They handed Castro a seven-year, $60 million contract in 2012, and subsequently committed $41 million over seven years to Rizzo.
The statement they were making with those contracts couldn't have been clearer. These are the guys they believe they can build a contender around, and that belief hasn't wavered.
While Castro suffered through a horrible 2013 in the field and at the plate, Epstein made it clear that he was not considering a position change or other solution to accommodate top prospect Javier Baez, a shortstop who hit 37 home runs last season.
We repeat: A shortstop who hit 37 home runs.
When Castro arrived at Spring Training, the Cubs made it clear to him and everyone else that he was still their shortstop, that they still believed in him. He's returning the favor with solid everyday play.
"It looks to me like he has settled down this year," a National League GM said. "He looks a lot better. He's always been a good player, but now he looks comfortable."
Castro, who hit a career-low .245 last season, was only 1-for-11 in the first three games against the White Sox, but he did take two walks Wednesday night. Perhaps that extra patience set him up for the two-run blast he sent into the left-field seats on Thursday. Castro has gone 14-for-48 with six extra-base hits in the last 12 games, raising his average to .280. He's hit five home runs and driven in 16 runs, producing a .761 OPS.
Rizzo is on a real roll. He was 3-for-5 with a walk in Thursday's blowout, although to be fair, he was lucky that the wind kept Moises Sierra from grabbing a flyball to the warning track that fell for a double. Rizzo's homer to straightaway center was his fourth in the last eight games, and seventh overall.
The White Sox had kept Rizzo quiet in the first three games of the series, but he's 10-for-28 with eight RBIs in his last eight games, raising his average to .294. He's driven in 18 runs to go along with the seven homers, producing a .926 OPS. Rizzo was laughing and having a good time on the bench late in the game -- hardly the demeanor he had throughout much of the Cubs' 96-loss season a year ago, when a .233 batting average made it easy to discount his 23 homers and 80 RBIs.
Hitting coaches Bill Mueller and Mike Brumley have done good work with both Rizzo and Castro, in particular helping Rizzo become comfortable at the plate late in counts. He entered the season with a .154 slugging percentage in two-strike counts, but has it at .375 so far this year.
It's important for the Cubs that Rizzo and Castro fulfill their potential as guys who should at least be in the All-Star discussion on a regular basis. They're taking a step in that direction in the first quarter of this season.
Through Wednesday, the Cubs ranked fourth among NL teams in OPS at shortstop (.750) and sixth in OPS at first base (.877). They were 12th and eighth, respectively, last year.
That's not the overnight turnaround that Cubs fans dream about, but it's progress. That's what this season is about for the Epstein, Hoyer and the Cubs -- playing the game better across the board.
Castro and Rizzo are doing their part.
Phil Rogers is a columnist for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.