Moustakas, 25, was the second overall pick in the 2007. He tore up the minors -- .999 OPS and 36 home runs in 118 games between Double-A and Triple-A in 2010, for example -- and was widely considered one of the 10 best prospects in baseball before the 2011 season.
In 40 games this season, Moustakas has hit .152/.223/.320 (47 OPS+) with four home runs. He is a career .236/.290/.379 (81 OPS+) hitter in 414 career MLB games. Moustakas hit very well in spring training (1.290 OPS) but proved that, once again, spring training stats are for suckers.
Utility man Jimmy Paredes has been called up to fill Moustakas' roster spot. The 25-year-old was hitting .327/.357/.486 in Triple-A and has a 61 OPS+ in 122 career games in the show.
Valencia, 29, has hit .308/.362/.423 (115 OPS+) with one homer in 15 games this year. He owns a 96 OPS+ in 351 career big league games.