Diaz spent the bulk of his career with the Braves, having played with them from 2006 to 2010. For the Pirates, he batted .259 with a .303 on-base percentage and a .324 slugging percentage, hardly good enough for a player who doesn't have much defensive value. He was acquired to hit left-handed pitching, and he didn't even really do that - yes, he batted .295 against southpaws, but it was an empty .295, with no power. He hit no home runs overall in 216 at-bats for the Pirates.
The Bucs signed four players to major-league contracts last offseason: Diaz, Lyle Overbay, Scott Olsen and Kevin Correia. Of those, only Correia is still with the team, and despite his All-Star appearance, not even he has pitched particularly well, with 77 strikeouts in 154 innings and a 4.79 ERA. The Pirates played surprisingly well for the first two-thirds of the season, which is remarkable given how badly their free agents played.
Adam Bittner adds:
With Andrew McCutchen, Jose Tabata, Garrett Jones, Xavier Paul and Alex Presley, the Pirates are reasonably well stocked in the outfield. Given the Braves' apparent willingness to take on Diaz's salary for next season, you probably have to score this as a win, even if the PTBNL ends up being Kevin James or something.
With Diaz out of the picture, two of general manager Neal Huntington's big free agent acquisitions over the winter have flopped miserably. First baseman Lyle Overbay was let go earlier this month to make room for Derrek Lee, leaving only pitcher Kevin Correia remaining in the free agent class of 2011.