The change will also convert Wilson Betemit, who has logged the majority of time at third base this season, to a backup role.
The promotion of Moustakas was no surprise. All along, the Royals had indicated that he'd be in Kansas City at some point this season, with June the likely arrival date. But the action came swiftly, with manager Ned Yost saying the decision wasn't made until after Thursday's 3-2 win over the Blue Jays.
"He's been hot, been swinging the bat well, driving in runs, playing a good third base -- so the time is right," Yost said.
Moustakas is batting .287 in 55 games for the Storm Chasers with 10 home runs, 15 doubles and 44 RBIs. He has a .347 on-base percentage.
Hosmer beat his buddy from Spring Training and Omaha to the Majors, arriving on May 6 to replace Kila Ka'aihue at first base after ripping off a .439 average in the Pacific Coast League.
"He's going to come into the lineup just like I did and do his role and not try to do too much," Hosmer said of Moustakas. "He's going to fit in perfectly with this team."
If Moustakas, a left-handed hitter like Hosmer, can match his pal, the Royals will be ecstatic. Hosmer has jumped into the No. 3 spot in the lineup and is hitting .309 with five homers and 21 RBIs in 32 games.
Yost said that Moustakas will become a lineup regular immediately, perhaps injecting new life into a team that has lost 14 of its past 19 games.
"Six days a week, five days a week, that's the minimum," Yost said. "We want him to get his feet wet. In the beginning, let him go ahead and play, get accustomed to big league life and let him go."
Moustakas will make his debut in the Los Angeles area where he was a star at Chatsworth High School. The Royals made him their first-round pick, the second overall selection in the 2007 First-Year Player Draft.
Hosmer said he'd phone Moustakas on the way to Anaheim, but he'd have no special advice for him.
"He doesn't need any advice from me," Hosmer said. "He's been through it all, and he's going to come up here and be nice and relaxed. The first game will be exciting for him in his hometown. I'm sure his family will be out there, and he's going to come up here and not miss a stride."
Aviles, struggling much of the season, has a .213 average with five homers and 30 RBIs in 47 games for the Royals. He also has a team-high eight errors at third base and second base.
"He's kind of stuck in a rut offensively and defensively," Yost said. "We know what kind of player Mike Aviles is. He's very athletic, a .300-type hitter, but it's been a struggle for him. ... He needs to take a step back and go down to Omaha and play every day. He's not going to have that opportunity here."
Aviles burst into prominence in his rookie season of 2008, proceeding from a big two-double day at Yankee Stadium on June 6 to a final .325 average in 102 games. But he had to undergo Tommy John surgery on his right elbow in '09 and missed most of the season. He returned last year to bat .304 in 110 games.
This season, however, has been a struggle for him.
"It's not like I've really been tearing it up or anything. I've pretty much stunk lately, haven't really helped the team at all," Aviles said. "It's not like it really came as a surprise. I've just got to get myself right and get back to normal."
Aviles played second base on Thursday, his 18th start at that position. He's also started 21 games at third base and one at shortstop.
Typically upbeat in the face of his demotion, Aviles expressed confidence he'd regain his batting stroke.
"Nobody wants to get sent down, but in all reality, sometimes it's for the best," he said. "I go down for however long, I come back and I'm raking again. We're not even worried about this little bit of time I'm gone."
His departure puts second base solely in the hands of Chris Getz, who's started 45 games there.
Betemit is batting .290 in 47 games and has shown improved defense at third base this season, but Yost also values him highly as a utility player.
Originally a shortstop, Betemit could fill in at that spot, as well as spell Moustakas at third base and Hosmer at first base if needed. Yost said he could also play left field.
"It gives us a little bit better bench and gives us a chance to get Moose up here and get his big league career started," Yost said.
A corresponding move to make room for Moustakas on the Royals' 40-man roster will be made on Friday.
Moustakas' credentials are impressive. Last year he batted .347 with 21 homers and 76 RBIs in 66 games for Double-A Northwest Arkansas and then had a .293/15/48 line in 52 games for Omaha.
Moustakas is 22 and becomes the ninth rookie on the Royals' 25-man roster.
"It's going to be awesome for him to make his debut in his hometown, and then we get back here and everyone in Kansas City hopefully comes out like they did for mine," Hosmer said. "He's a great player and everybody should be excited for him."