For Monday, they were tourists, taking in the sights during an off-day for Double-A Erie in nearby New Hampshire. The next time he gets to a big league park, it figures to be for real.
"There's been people like these guys saying, 'Get used to it. It's your locker in September,'" Castellanos said. "It's awesome to hear these guys say it, but I know that it's not real until it happens. I'm definitely taking this all in, but I definitely realize that it's a visit. It's not because I deserve to be here."
Castellanos, who is the Tigers' top ranked prospect by MLB.com, is batting .309 with six homers and 19 RBIs in 47 games with the SeaWolves.
The visit was an idea on their part after an all-night bus ride from Erie. When they checked the schedule and saw the Tigers were in Boston, they made some calls, got some tickets from Brayan Villarreal and found a driver willing to make the 40-minute trek, as long as the fare was paid up front.
They chatted with Miguel Cabrera, Justin Verlander, Prince Fielder, Ramon Santiago and Delmon Young. They hung out in the dugout, on the field and behind the cage during batting practice, and they caught the game from the stands.
All the while, you had to figure what they were thinking: This could be them soon. In Castellanos' case, it could be sooner rather than later.
"I want to get to the big leagues as soon as possible," Castellanos said, "but if these guys think Triple-A is necessary, then that's something that I don't control, nor would I try to."
At this point, he doesn't know where his season is going to end once Erie wraps up its campaign the first week in September. He's almost certain to be somewhere doing baseball work in September and October, whether it's Detroit or the Arizona Fall League or getting ready for winter ball.
"That's a possibility," said Castellanos, who was named the Futures MVP after a three-hit, three-run, three-RBI performance to help the United States team cruise to a 17-5 win over the World squad. "I've heard instructional league's a possibility. I've heard if we make the postseason and I keep hitting off lefties, I stay with the team to the postseason. So everything's a possibility. We're just going to have to wait and see what happens."
He has also heard more than enough trade rumors for his liking.
"Last year, I was able to avoid it," he said. "This year, no. This year, it's been every day, all the time, in my face. I'm definitely ready for the Trade Deadline, so I can just relax."