He played in Major League Baseball (MLB) for the Cleveland Indians, Tampa Bay Rays, Philadelphia Phillies, Los Angeles Dodgers, Houston Astros, and Atlanta Braves.

Hernández was a regular part of the Indians' starting rotation for six years under the name of Fausto Carmona, which he had used to obtain a visa to pitch in the United States.

[6] In the span of seven days (from July 30 through August 5), Hernández recorded four losses and three blown saves for the Indians, including walk-off home runs surrendered to Boston's David Ortiz and Detroit's Iván Rodríguez.

After the closer experiment, Hernández was demoted to Triple-A Buffalo on August 26, where he was placed back in a starting role.

[11] In 2007, Hernández was originally slated to be the Indians' spot starter and to begin the season with the Triple-A Buffalo Bisons.

[13] Hernández quickly rebounded, however, holding the New York Yankees' potent offense to just two runs in six innings in his next start on April 19.

During this streak, Hernández beat two-time American League Cy Young Award winner Johan Santana of the Minnesota Twins twice and pitched his first career complete game shutout.

[15] After one of these wins, Minnesota center fielder Torii Hunter was quoted as saying, "I can't wait until we face normal pitchers.

[17] On October 20, Hernández was battered by the Red Sox at Fenway Park in Game 6 of the 2007 ALCS, surrendering seven runs in two plus innings, including a first-inning grand slam by J.D.

[19] Hernández received serious consideration for the AL Cy Young award, along with teammate CC Sabathia, Boston's Josh Beckett, and the Angels' John Lackey.

[11] On April 10, 2008, the Indians signed Hernández to a four-year $15 million contract extension through 2011 with three individual club options through 2014.

[22] Hernández was placed on the disabled list the next day,[23] where he remained for two months before making his return to the starting rotation on July 26.

Sheffield again started exchanging words with Hernández, and this time charged the mound inducing a bench-clearing brawl.

Hernández and Sheffield were both ejected, along with Indians catcher Víctor Martínez and Tigers second baseman Plácido Polanco.

[11] On February 28, 2011, Indians manager Manny Acta announced that Hernández had been selected as the starter for Cleveland's season opener against the Chicago White Sox.

[28] In front of a sellout crowd of 41,721 fans at Cleveland's ballpark Progressive Field, Hernández started the 2011 season for the Indians.

Hernández recorded his first career win under his real name on April 21, allowing one run in six innings while striking out seven in an 8–1 victory over the Oakland Athletics.

[42] Around that time, Phillies analyst Ricky Bottalico commented, "All in all, I like what I've seen from Roberto Hernández early in the ball games, but it's seems to be fourth, fifth, sixth innings, you're seeing a laboring guy out there.

[49] On December 18, 2015, Hernández signed a minor league contract with the Toronto Blue Jays that included an invitation to spring training and a March 28 opt-out.

He did not throw exceptionally hard when he debuted in the minor leagues at age 21, but developed a fastball that could reach 97 mph.

In January 2012, Dominican police arrested Hernández after he left the U.S. Consulate, accusing him of using a false identity to obtain a visa.

[34] While he was known as Fausto Carmona, his birth date was given as December 7, 1983, which would have made him 22 years, 4 months old at the time of his MLB debut.