[1] Alvarez, who is of Mexican American descent, attended Fontana High School and Long Beach State University, where he posted a 23–5 record with a 2.56 earned run average in three seasons.

He returned to Long Beach State in 2010 to complete a bachelor's degree in history and served as the undergraduate assistant pitching coach for the university's baseball team.

[2] Signed by the Boston Red Sox in 2003 after being drafted 49th overall, Alvarez made his professional debut with the short-season Single-A Lowell Spinners.

In 2004, Alvarez was named Pitcher of the Year for Double-A Portland Sea Dogs after going 10–9 with a 3.59 ERA in 26 starts in his first full pro season.

He limited opposite hitters to a .252 average, posted a 3.4 strikeout-to-walk ratio (108-to-32), and was also named to the Eastern League's mid-season All-Star team.

The Red Sox summoned Alvarez on July 22, 2004, from Double-A Portland when a spot starter was needed for a doubleheader against the Baltimore Orioles.

[3] On July 2, 2005, Boston placed reliever Matt Mantei on the 15-day disabled list with a left ankle ligament sprain and recalled Alvarez from Triple-A Pawtucket Red Sox.

At the time of promotion, Alvarez was 7–3 with a 4.23 ERA in 16 starts for the PawSox, including wins in five of his past six decisions.