This was, not unexpectedly, the primary pregame topic at Nationals Park, and comparisons between the two players were inevitable, even though very possibly misplaced.
"It's not like he's a pitcher. He's an outfielder, we can't really compete against each other," Trout said of Harper. "We're both trying to get hits. For my game, when I try to do too much, that's when I get in trouble."
On the topic of comparisons with Harper, Trout said: "I kind of figured it. Same age, same hype. We're always going to be compared. It's pretty cool getting compared to not just Harper and stuff, but to Hall of Fame guys."
When asked a similar question about comparisons with Trout, Harper replied: "I really don't care. I couldn't care less about opinions. If they like him, they like him. If they like me, they like me. If they like both of us, then they know the game. If they don't, they're crazy.
"This game's a lot of fun to play. We're both human. We're both going to make mistakes, make errors. That's part of the game, part of our lives. It's fun to go out there and play. People blow things up to bigger proportions. That's how this world is now. That's how it is."
Asked if he compared his statistics with those of Trout, Harper said: "Not at all, because I know I'm a damn good player. He is, too. We're going to roll through baseball for the next 20 years, hopefully, and make people turn their heads. He's going to do it, and hopefully I am, too."
Harper is 21. Trout is 22. Both were Rookie of the Year Award winners in 2012. It is not as though any differences they have can be explained by generational means. Harper is an edgier character than Trout. Trout is, as a public person at least, the more genial of the two.
These comparisons, of course, may be easier for Trout, because he has enjoyed the larger level of success. He is operating on a level seldom reached by any player.
The veteran Raul Ibanez, whose ninth-inning, pinch-hit three-run double made the difference for the Angels on Monday night, said: "Mike Trout is probably as good a player as the world has ever seen."
The contemporary player Trout is most often compared with is not Harper, but Miguel Cabrera of the Tigers.
The last two seasons have been filled with these comparisons. Advanced metrics have indicated that Trout is the best player in all of baseball. The Baseball Writers' Association of America has in both 2012 and '13, named Cabrera as the Most Valuable Player Award winner in the American League.
Harper and Trout praised each other repeatedly and with apparent sincerity on Monday. But the idea of comparing anybody, other than Cabrera, to Trout, is probably not a useful exercise. And this does not mean that Harper will not achieve his own kind of greatness.
"I don't think it's fair," Nats manager Matt Williams said of comparisons between Harper and Trout. "Everybody brings a different set of tools to the table. Bryce is one of our main guys. We rely on him. And the Angels rely on Mike.
"The comparison is natural, I think, because of all that's been written and all that's been documented, and all that. But it's probably a little unfair to compare them, because they're two very different players. They both have exceptional tools and exceptionally high ceilings. But to compare them, I don't think Bryce compares himself to Mike, or vice-versa. And we certainly don't do that."
Halos manager Mike Scioscia essentially voted no on the Trout vs. Harper storyline.
"This series is not Mike Trout vs. Bryce Harper," Scioscia said. "Those are two guys who are in a different class of player than you're going to see, and you get a chance to see them on the same field, but they're both going to do what they can do to help their team win. That's what it's about."
Scioscia was on the right track there. Neither Trout, who had two singles in five at-bats, nor Harper, who was 0-for-3 with a walk, was a dominant figure on this night. But with these two, the possibility of greatness is ever-present.
One way or another, when two players who represent the future of baseball have their paths intersect, there is bound to be considerable attention paid, and yes, comparisons made. They are different people, but Trout and Harper have the kind of ability and competitive fire that make this joint appearance a truly special occasion.