It has all happened swiftly for the right-handed hitter, who signed a $72.5 million contract Aug. 23, and has since played in playoff games in the Gulf Coast League, the Eastern League and the International League.
With each step he takes, Castillo realizes his dream is getting closer.
"Obviously, the competition's higher," said Castillo through an interpreter. "There's more veteran players, but I'm still trying to play my game."
On Tuesday, that game Castillo spoke of consisted of a 1-for-4 performance. The hit was a rocket single off the glove of Durham third baseman Cole Figueroa and into left field.
Castillo, whose top tool is probably his speed, was then caught trying to steal.
But that didn't dampen his competitive spirit in the least.
"It didn't catch me by surprise, that's part of the game," said Castillo. "If I had gotten on again, I was going to try to steal again."
And the PawSox weren't going to stand in his way.
"We're letting him play," said Pawtucket manager Kevin Boles. "We're letting him play. He's going to get his at-bats, we're going to watch him, we're going to feel him out as far as his positioning and defense as far as him running the bases.
"We'd rather him be aggressive. We're just going to let him play and see how he fits in here and he's done a nice job. He had a real good workout today and played a good game."
Even on quick glimpse, there's a lot to like about Castillo.
"[He has] plus hand speed through the zone, he expanded the zone a couple of times, but had the well-hit ball to third base," said Boles. "That ball got down the line, got to the third baseman, it was on him really quick. But he's an athlete. You can tell. There's no doubt about it. He's a fast-twitch body and there's a lot to be interested about there, yeah. It stands out."
Pawtucket defeated the Bulls, 3-2, to take a 1-0 lead in the best-of-five series. If it goes the distance, Game 5 would be Saturday.
After that, Castillo could join the Red Sox.
"You know, it's human nature to think about the big leagues and playing for the Red Sox at some point," Castillo said. "I definitely focus at every level that I'm at. I have my mind on the moment and improving and getting better."
One of the main things Castillo tries to do each day is simply shake off the rust. Remember, he had been away from competitive baseball since July 2013.
"Timing is pretty good," said Boles. "He was aggressive. He's looking to impact the baseball, there's no doubt about it. But it's a first look and watching him go through his routines today, there's a lot to like here. There's a reason why we signed him. It's a pretty athletic body right there, he's impressive."
"Obviously it's been a gradual step from one level to the other and it's helped me find my game, which obviously I haven't played [in over a year]," Castillo said. "It's been extremely beneficial just to find myself as a player again."
The fascinating thing is that Castillo knows who that player is, but most of the people who watch him don't.
All there is to go by is some brief video clips from his days in Cuba and some glimpses from his at-bats in Boston's farm system.
At some point, Castillo will take in the new culture that surrounds him. For now, he's trying to keep it low key.
"I stay in the room, I love music, and I'm learning the music of this country," he said. "Nothing else really sticks out -- just talking and meeting you people."
Tuesday marked the first time Castillo played all nine innings since starting up a couple of weeks ago.
"It felt good," Castillo said. "I had the gradual progression plan that the team put me on and built up to this moment, so I played seven innings last time out, so I felt good."