Nobody could've predicted he'd have this hot of a start to his Chicago career when the Cubs traded for him at the 11th hour of the deadline last month.
Theo Epstein's front office and Joe Maddon's clubhouse expected a professional hitter, but a .392 average and 1.214 OPS in his first three weeks is beyond anybody's wildest dreams. More than half of his games in a Cubs uniform (10) have been of the multi-hit variety.
He's also giving his team early leads on a seemingly regular basis. Wednesday night, he became the first player in Cubs history to hit a first-inning home run in three straight games when he took Dereck Rodriguez deep for a two-run shot.
But even beyond the box score, Castellanos is having a huge impact.
It isn't just the passion, though that is palpable on a nightly basis:
Kris Bryant - who hit the homer that Castellanos is celebrating in that highlight - joked he thought the Cubs outfielder actually hurt himself and that's why he was jumping around.
"I saw that, I thought he, like, sprained his ankle or something and he was just jumping 'cause it hurt," Bryant laughed.
But that energy has been infectious for this club, from his "every day is Opening Day" mentality to his hustle.
In Wednesday night's wild 12-11 victory over the Giants, Castellanos beat out an infield hit - his fourth knock of the contest - which set the stage for Bryant's heroics.
"He's been really, really good for us. He beat that out hustling down the line," Bryant said. "If that didn't happen, who knows what sequence of pitches I would've got. It could've changed the whole sequence. It could've changed the whole inning, so a lot of credit to him."
On two instances in that game, the other two Cubs outfielders (Kyle Schwarber, Jason Heyward), each beat out the backside of a double play ball, with Schwarber's directly leading to an important run in the sixth inning.
"That's been going on a lot lately," Maddon said of the hustle plays.
That can't all be tied to Castellanos. Schwarber hustled out a key grounder in a game in Milwaukee late last month just a few days before Castellanos was acquired and Bryant's hustle down the first-base line has been a well known staple of his game from the moment the Cubs drafted him.
But it certainly never hurts to add another such high-energy guy into the lineup, and Castellanos has been like that from Day 1.
Maddon said he went to Heyward and Schwarber during the course of Wednesday's game to let them know he recognized their effort and appreciated it. The manager whose only rule is to "Respect 90" tries to instill that mindset on Little Leaguers and kids - "It doesn't take any talent to do that."
"It leads to a lot of runs," Maddon said. "...That's the kind of stuff that goes unnoticed. All good. You win 1-run games because of that, even if it's 12-11. We've been doing that a lot and I really appreciate it."
Then there's the trickle-down effect of what Castellanos' arrival has done for the Cubs lineup.
"When you have the type of guy that is capable like Castellanos is to change a game from an at-bat, that's huge," Cole Hamels said. "He seems to be on second base all the time, so that helps out, turning the lineup over, getting runs in and really giving KB and [Anthony] Rizzo something to drive in. That's great for us to be able to have that.
"Plus, it makes the lineup deeper. As a starting pitcher, when you see the type of lineup we're putting out there, that's a tough lineup. It doesn't make things easy at all. ... The energy he's bringing has been outstanding. In the clubhouse, he's fit right in."
Imagine how deep the lineup will look like with Willson Contreras returns, as the All-Star catcher has been doing strengthening exercises on his injured hamstring the last couple days.
Not only does Castellanos' on-base and extra-base hit prowess put him in scoring position for the heart of the Cubs lineup, but hitting directly in front of Bryant can have a positive effect on the looks the former MVP gets before he even steps into the box.
"We are very similar hitters," Bryant said. "Pretty close to the same swings, too, so I get a lot of information from his at-bats and he works a lot of counts. Just a really professional hitter and he's so underrated. He's definitely making a name here for himself and we couldn't be happier."
It's impossible to quantify the little things Castellanos does or how much different the Cubs would look without him right now, but the calls to extend him from fans and even media have begun to pick up steam and it's understandable why.