Like Harper, Upton also made his Major League debut at the age of 19, though the D-backs outfielder was just a few weeks shy of his 20th birthday when he got the call in 2007. Harper will not turn 20 until Oct. 16.
Physically, Upton was ready for the jump, but there was still much to learn about the subtle intricacies of the game. There were also the mental challenges that come with struggling for the first time on a baseball field.
"Obviously to get to this level, you have to be confident in your abilities physically and what you can do on the field, but there's a mental side of it too that takes a while to really get a grasp of," Upton said. "You never learn everything in this game, but once you get a grasp how to deal with a 162-game schedule physically and emotionally, you start to hone in on your abilities.
"There will be times this game will stress you out and your abilities aren't able to really flourish on the field. When you're able to harness that and refocus yourself when times aren't great, that's when the game comes to you."
Upton was an All-Star in 2009, and he had his finest professional season in 2011, when he hit .289 with 31 homers and 88 RBIs.
Like everyone else in the game, Upton will be interested to see how Harper's career progresses.
"He's obviously a very talented player and he's going to play for a long time," Upton said. "I think until you realize that, 'Hey, I'm here to stay, I belong,' it's a bit of a transition, but he seems to have a pretty good feel for the pressure and expectations. He doesn't seem like the kind of guy who is going to let it get under his skin, so he'll be fine.
"Obviously he's going to struggle at some point, and that's when you really find out what you're made of. That's when I figured out what I was made of. It's a fun ride and I think he definitely seems ready for it."