He didn't see another fastball until he got to the opposing bullpen.
"I've had to learn a lot," Castellanos said earlier, "because [at] Triple-A, batting 3-4-5 -- which I've done all year -- you don't see a lot of fastballs for strikes. They pitch around you. I've had to learn at this level to hit offspeed pitches more so, because they throw them for strikes more consistently.
"At the lower levels, [the pitchers' strategy] was get ahead with a fastball for a strike and then try to get you to chase with an offspeed for a strike and then come back with a fastball. Now, they'll just show you fastballs around the zone to keep you on it and then they'll throw you offspeed for strikes."
This doesn't surprise his manager. At 21, Castellanos remains one of the youngest players at the Triple-A level, but Phil Nevin expected him to adjust at the plate.
Monday marked Castellanos' 15th home run for Toledo. His 134 hits ranked third in the International League entering Tuesday, while his 34 doubles were tied for first. His .794 OPS ranked 15th.
Castellanos was hitting .250 on May 23 and .300 on July 1. His batting average has found a middle ground in the .270s.
"There's not a level high enough where Nick's not going to hit," Nevin said. "He's going to hit."
There's only one level up remaining for Castellanos. The only question left is when he'll get there.
If it's going to be this year, the Tigers have a couple of weeks left to make a move. If it's going to include October, he has to be summoned this month.
A year ago at this point, the Tigers were debating whether to call up Castellanos or Avisail Garcia for the stretch run. Both were at Double-A Erie at the time. Castellanos had just converted from third base to the outfield, so his path wouldn't be blocked by Miguel Cabrera. His obstacle that summer became Garcia, who could play all three outfield positions, pinch-run and play defense.
Garcia, called up at the end of August, became a late-season hero and ended up starting in right field in the World Series. As late-season talent infusions go, he was the best-case scenario. But the Tigers were three games back when they called him up. They needed an infusion.
Detroit, which traded Garcia to the White Sox last month, entered play Tuesday needing better play from its left fielders. But it also entered Tuesday 6 1/2 games up in the American League Central.
Castellanos doesn't deny that he thinks about the possibility. When he does, though, he's also thinking about it from Detroit's standpoint.
"The reality is they're seven games above, [Andy] Dirks is starting to get hot, [Matt] Tuiasosopo has been consistent all year, and they're winning," Castellanos said. "The way sometimes this game works, if they don't need me, why are they going to bring me up? If they don't need to start my [service time], if they don't need to do anything?
"Now if they were one or two games back and if people weren't swinging well or whatever, then maybe I could see that. Of course, I'd love to go up. I'd love to try to help the team win. But I'm not walking by the manager's office expecting something to happen."
So for now, when asked about his plans for September, Castellanos is thinking about his newborn son, Liam, back home in South Florida. His Major League debut, as close as it seems physically and figuratively, he's keeping at a distance.
"The way I'm looking at it right now is I'll be home with my son in 14 days," Castellanos said Monday night from Fifth Third Field, "and if anything else changes, that's definitely for the best."
His manager, meanwhile, is leaving that decision to the organization. When Nevin talks about the way Castellanos has made himself into an outfielder with extra work almost every day, how he has learned to think with Triple-A pitchers, it's clear how much he'd like to see the kid eventually get a chance.
"He should be coming out as a junior in college. To do what he's done each day at this level, at that age, is impressive," Nevin said. "I'm tired of people trying to compare him to [Mike] Trout and [Bryce] Harper and guys that have come up early. There's different development for different guys. He's going to be just as good as these guys, in my opinion."
Then he paused.
"We look at things in the long haul," Nevin said.