Infielder Cheslor Cuthbert also was optioned to Triple-A and Matt Strahm was sent to Double-A.
That leaves Travis Snider, Whit Merrifield, Reymond Fuentes and Terrance Gore to battle for one or two backup-outfielder spots behind Alex Gordon, Lorenzo Cain and Paulo Orlando (likely the right-field starter).
Fuentes seems likely to be the fourth outfielder, and Merrifield is inching close to perhaps being the club's 25th man because of his versatility. Merrifield can play infield and outfield.
The Royals reassigned pitchers Christian Binford, Jonathan Dziedzic, Aroni Nina, Brooks Pounders and Sam Selman to Minor League camp, along with infielders Hunter Dozier and Balbino Fuenmayor and catchers Zane Evans and Cam Gallagher.
The Royals also optioned pitchers Miguel Almonte, the team's No. 4 prospect according to MLB Pipeline, and Kyle Zimmer, the team's No. 2 prospect, to Triple-A Omaha, along with Eibner and Martinez.
Right-hander Alec Mills, infielder Ramon Torres, Bonifacio and Starling were optioned to Double-A Northwest Arkansas.
Of the cuts, the only real surprise was perhaps Martinez, who had a shot for the final roster spot after winning the Pacific Coast League batting title (.384) last year. Martinez was having a decent camp (.278) as well.
"Right now, it starts to get tough to get those guys regular at-bats," manager Ned Yost said. "We need guys like Eibner and Martinez getting regular at-bats. Instead of getting four at-bats in five days, they'll get four at-bats every day down there [in Minor League camp]."
Zimmer has also had a solid camp, posting a 1.50 ERA in three appearances.
"I like the fact he was out there pitching and wasn't hurt," Yost said.