Rivera, who is 6-feet-2, 195 pounds, hit .310 with 12 home runs, 27 doubles and 72 RBIs for Class A Lexington this season.
"He's a physical kid, big and strong," Royals assistant general manager J.J. Picollo said. "He's got really good presence. He plays beyond his years. The way he walks and carries himself, it's like watching a Triple-A player."
The Royals see a bright future for him.
"He's got power, you see that right away," Picollo said. "And I think there's more in there. And he hits the ball the other way. Good defender, too."
Griffin, 22, was a high-school first-round pick in 2014 out of Orlando, Fla. The 6-foot-3, 200-pounder went 15-7 this season with a 3.35 ERA combined between Class A Wilmington and Double-A Northwest Arkansas. Griffin also represented the organization in the Futures Game in July.
Griffin's 2017 season, his fourth as a professional, was his best.
"More than anything, he sped up his delivery this year," Picollo said. "His fastball got better. His command got better. His curveball got better. Then his confidence improved."
Griffin's fastball sits in the 90-91 mph, though he can touch 93.
"It's nothing overwhelming, but he works his pitches in and out and now he's working up and down in the zone," Picollo said. "He's still at the age where there may be more there. He may get stronger as he gets older. He strikes us as a guy who won't rely on his velocity, though."
Each team's Hitting and Pitching Prospects of the Year were chosen by the MLBPipeline.com staff. To receive consideration, players must have spent at least half the year in the Minors, appeared on the team's Top 30 Prospects list and played the entire year in the organization.