Moss, 28, was batting .286 with 15 home runs -- second most in the Pacific Coast League -- and 33 RBIs in 51 games for Sacramento. He hit four home runs in his past five games for the River Cats.
According to Melvin, Moss had a clause in the Minor League contract he signed with Oakland that would have allowed him to become a free agent if he hadn't been promoted by June 15. The A's needed to open a spot for Moss on their 40-man roster, prompting the decision to designate Ka'aihue for assignment.
Moss was in the starting lineup Wednesday night against the Texas Rangers, batting seventh and playing first base. He went 0-for-3 in the A's 2-0 win but made a couple of nice plays, including digging out a long one-hop throw to retire Adrian Beltre in the sixth.
"He's been playing [first base] for a while [at Sacramento]," Melvin said. "And if you look at the numbers, a guy with a lot of power. We do need guys here with power. I think one of the reasons [for] the timing of it is he did have an out on the 15th of June, and based on the fact he was doing what he was doing and a little bit of the lack of production that we've had here, we felt it was the right thing to get him here.
"It was very difficult to let Kila go. Sometimes you just have to make tough moves, and today was one of them."
Ka'aihue batted .234 with four home runs and 14 RBIs in 39 games with the A's.
Brandon Allen, Daric Barton and Ka'aihue each tried but failed to hold onto the starting job at first base. Now Moss will get a chance, and all eyes will definitely be on him when he's in the field.
"Obviously I'm not a Gold Glove first baseman, but I've played there a good bit," Moss said. "I played there in winter ball a good bit and I played there for a few years in the Minor Leagues with the Red Sox, so it's not completely foreign to me, but I'm not going to be Daric Barton defensively over there, but hopefully I do well. I think I will."
Moss has spent time with Boston, Pittsburgh and Philadelphia. He's a .236 career hitter with 15 home runs and 78 RBIs in 249 games.
Melvin said that delivering the news to Ka'aihue was rough, in part because the first baseman's wife is pregnant with twins and expected to give birth soon.
"He's a class act and I wish him the best," Melvin said. "If he doesn't end up being picked up by somebody else we certainly would like to have the chance to potentially see him here again. You have tough conversations like that over the course of the season and that was one of them."