Diaz logged just 11 at-bats with New York this season, batting .182 (2-for-11) with two RBI in six games. Last year, he played in 89 games.
At Triple-A Norfolk, Diaz struggled, never regaining the form that led many to draw comparisons to a young Manny Ramirez earlier in his career. In 103 International League games, Diaz batted .224 with eight home runs and 38 RBI, striking out 99 times in 379 at-bats.
His best month was June, when he batted .284 (33-for-116) with five home runs and 19 RBI, but he had been slumping for some time on a poorly-performing Tides club, batting .171 in July and .190 in August.
Diaz was designated for assignment last week to make room for outfielder Shawn Green on New York's 40-man roster.
In Colorado, where the Mets won for a second consecutive day by a large margin, Mets manager Willie Randolph said he wasn't surprised by the trade of Diaz.
"He was going up and down focus-wise," Randolph told reporters. "I told him every time I sent him out that I wanted him to go down and focus and play hard and hustle and all of those things."
Nickeas, 23, was a fifth round selection of the Rangers in 2004 from Georgia Tech. Considered a strong defensive backstop, Nickeas will report to Class-A St. Lucie of the Florida State League, but had been playing at Double-A Frisco of the Texas League.
In 39 games for Frisco, Nickeas was batting .248 (28-for-113) with two home runs and 15 RBI. He opened the season at Class-A Bakersfield of the California League, batting .297 (19-for-64) with no home runs and six RBI in 17 games.