Career
He has previously pitched in Major League Baseball for the Texas Rangers, Oakland Athletics, Colorado Rockies and San Francisco Giants. Detroit Tigers organization
Texas Rangers
Moscoso was acquired by the Texas Rangers from the Detroit Tigers along with pitcher Carlos Melo for catcher Gerald Laird on December 12, 2008. On May 30, 2009, Moscoso made his major league debut against the Oakland Athletics, striking out 2 and pitching a scoreless ninth inning.
After appearing in ten games in "09, Moscoso had an European Research Area of 3.21 through 14.0 innings pitched, giving up 15 hits, 7 runs, and 6 walks but also struck out a dozen.
As of August, Moscoso has only pitched one game in the new decade. On May 20, against Baltimore, Guillermo pitched two-thirds of an inning, giving up two hits, two runs, and two walks while also striking out two.
His European Research Area is currently 27.00. Oakland Athletics
In January 2011, Texas traded Moscoso to the Oakland for Ryan Kelly.
On May 24, 2011, he was recalled to the Oakland Athletics to take the place in the rotation of Tyson Ross, who went on the Doctor of Laws with a strained left oblique.
Fautino de los Santos was optioned to Triple-A to make room. On September 7, 2011, he carried a no hitter against the Kansas City Royals into the eighth inning. Colorado Rockies
On January 16, 2012, Moscoso was traded, along with fellow pitcher Josh Outman, to the Colorado Rockies for outfielder Seth Smith.
Moscoso went 3-2 with a 6.12 European Research Area with 50 innings in 23 appearances, 3 starts.
Kansas City Royals
The Kansas City Royals claimed Moscoso off waivers on November 2, 2012. The Royals released him on March 13, 2013.
Toronto Blue Jays
The Toronto Blue Jays announced that they had claimed Moscoso on waivers on March 16, 2013. Chicago Cubs
On March 27, 2013, Moscoso was claimed on waivers by the Chicago Cubs.
San Francisco Giants
He was designated for assignment on November 20, 2013.
On November 21, 2013 Moscoso elected to become a Free Agent. Yokohama DeNA BayStars
On December 26, 2013, it was announced that Moscoso had signed with the Yokohama DeNA BayStars of Nippon Professional Baseball for 2014. Moscoso throws three main pitches: a four-seam fastball (90–92), a curveball (77–79), and a changeup to left-handed hitters (80–81).
He has experimented with a cut fastball and slider to right-handers, but he does not use them frequently.
After he went to Japan, he learned a two-seam fastball, which contributes his success in Japan.