"We feel the time is now," Royals general manager Dayton Moore said.
Hosmer hit .439 (43-for-98) for Omaha with an on-base percentage of .525. He had five doubles, three home runs and 15 RBIs in 26 games.
Manager Ned Yost said Hosmer will go into the lineup immediately.
"He's in a league of his own, the time is right," Yost said.
The promotion comes sooner than expected. The Royals had expected to have Hosmer get about 200 or 250 at-bats in the Pacific Coast League before calling him up. But his hot start accelerated their plans.
"He's ready, it's as simple as that," Yost said. "The kid's hitting .440 and he's ready to come up."
Ironically, third baseman Mike Moustakas had been expected to be the first hitting prospect promoted this season. But Moustakas is off to a slow start for Omaha, batting .235, although he has 20 RBIs along with four homers. He and Hosmer are ranked sixth and seventh, respectively, on MLB.com's Top 50 Prospects list.
Besides, Wilson Betemit and Mike Aviles have been holding down third base and hitting well. First base was another matter.
"We're playing pretty good baseball, and we need some production out of that first-base spot," Yost said.
Indeed, the Royals' winning start -- they're 17-14 and an early contender in the American League Central -- added impetus to the Hosmer move. There's a growing feeling that the Royals could be a big factor in the race this year.
"That's the general consensus," Yost said. "I don't want to say we're playing better than we anticipated, because we came into this thing anticipating playing pretty well. And we've played very good baseball from day one of Spring Training."
To make room on the 40-man roster for Hosmer, the Royals shifted catcher Jason Kendall from the 15-day disabled list to the 60-day DL.
Ka'aihue began the season as the primary first baseman, with Billy Butler moving to the designated-hitter slot. But Ka'aihue's average is down to .195 in 23 games, with just six RBIs and two homers.
"We need to get Kila going," Yost said. "Kila's an important part of our organization. We're going to need him to be productive. He'd been fighting it up here. It gives him a chance to re-group down there and get his swing back in line. We're going to need him sometime over the course of the year to give us a big push."
Not that long ago, it was Ka'aihue who was Kansas City fans' flavor of the month, after he blasted 37 homers in the Minors in 2008. But he never caught fire in his big league chances.
"At first base, you need to have prototypical-type numbers. You need to have guys that can drive in runs and that can hit for average, and Hosmer's hitting .440 down there. Everybody agrees watching him that he's ready," Yost said.
Moore said that the Royals have had their scouts evaluating Hosmer all season.
"We've had pretty much all our scouts in Omaha, or seeing him on the road, pretty much every game they've played and, to a man, everybody comes back and says, 'He's ready and we need him on our Major League team to help us continue to win," Moore said.
Left fielder Alex Gordon was among the Royals players who were impressed by the 21-year-old Hosmer's highly-advanced hitting and fielding prowess in Spring Training.
"Unbelievable, he was a freak," Gordon said.
Right fielder Jeff Francoeur expressed regret about Ka'aihue's departure, because of the hard work he put in, but he couldn't dispute that Hosmer was ready.
"You kind of know where we're heading offensively," Francoeur said. "We're all swinging it, and you put that in our lineup with what we've have now, you've got some stuff going on."
Hosmer warmed up in the Cactus League with a .450 average, along with two homers and eight RBIs in just 20 at-bats. He also showed exceptional fielding skills at first base.
"We got to see what kind of a kid Eric is in Spring Training, so we're not worried about him coming up here too fast. We feel like he's going to fit in right away," Gordon said.
Although Hosmer has hit just three homers so far this year in Triple-A, he had a total of 20 last season for Double-A Northwest Arkansas and Class A Wilmington. Moore believes his home-run power will increase as he matures.
"He takes his walks and gets himself in good counts. Power is something that comes as players mature, and his swing is through the middle of the field. That's where his approach is, and that's why he's hitting high for average," Moore said. "But there's no doubt in our minds that he has a lot of power long-term to drive in a lot of runs and be a power producer on the corner."
His Major League time begins on Friday night.
"He can flat out hit," Kendall observed. "He handled himself like a big leaguer. It'll be fun to see him get a shot."
That's what Kansas City is counting on.
"We're doing good, the time's right, he's coming," Yost said.