He was a 5-foot-11 outfielder who could barely hit his weight. Maybe 185 pounds in baseball spikes. But not much more than that.

Oh sure, Kyle Zimmer had some tools. La Jolla High School baseball coach Gary Frank can tell you all about those. First off, there was the rocket arm, the one Zimmer used to show off in the outfield or at third base. There was some athleticism, too.

It made sense. Zimmer came from good stock. His dad, Eric, had been a college baseball player at UC San Diego. And his mom, Cathy, had run track at San Diego State. So, yes, some of that natural ability always came to the surface while Zimmer was getting a callup to the varsity as a sophomore or excelling in water polo during his underclassmen years. But there were realistic expectations, too. Maybe Zimmer could earn a scholarship to a small school as a third baseman. That was his focus, Frank said.

But how about this? On Monday evening, Frank stood in the Zimmer home in the San Diego area and watched as his former third baseman — the kid who hit .200 as a junior — was selected No. 5 overall by the Royals in baseball’s draft … as a right-handed power pitcher.

“He didn’t throw a single inning as a junior,” Frank said. “I knew he had potential, but I’m surprised that he had this much improvement so fast.”

On Monday night, after Zimmer’s aunt had pushed forward past a crowd of family and friends at the watch party and tossed a Royals hat to her nephew, it was easy for Zimmer to tell the story of his transformation to the mound.

“I was always a hitter growing up,” Zimmer said. “But I guess the move paid off.”

These days, Zimmer stands 6-feet-4 and 225 pounds, the product of two late growth spurts. But his transition to pitching didn’t begin until he arrived at the University of San Francisco. The Dons, Zimmer says, had an incumbent third baseman who was blocking his path. So a coach suggested that the strong-armed Zimmer give pitching a shot.

It took some time — Zimmer threw just 90 mph with some raw mechanics — but he was soon sitting in the mid 90s with a wipeout curveball.

“I didn’t ever think I was going to be pitching and doing this,” Zimmer said. “But it’s something I’ve grown to love, and I’m glad I made the transition.

“My sophomore year, coming into that season, I knew I had the frame and the God-given abilities to get it done. I thought with my competitive attitude and my work ethic… I never really put a limit on myself.”

By late last season, Zimmer started popping up on radars after outdueling future No. 1 pick Gerrit Cole of UCLA in an NCAA regional. He followed that up with a breakout performance in the Cape Cod League — and the Royals’ scouting department started paying close attention.

“The scary part for us is we think he’s got huge ceiling left in him,” said Royals scouting director Lonnie Goldberg, who compared Zimmer to Giants starter Matt Cain. “You’re looking at a fresh arm — a young kid. He’s only 20 years old.”

And by some twist of fate, Zimmer’s late start on the mound may have provided the ideal foundation. “We’re getting a guy who we think is already pretty polished,” Goldberg said. “But we’re also getting a guy who we think has a ton of ceiling left in him.”