Diaz signed a two-year contract with Pittsburgh last December as a free agent after spending five seasons with the Braves. He was hitting .259 in 100 games for the Pirates this year.
The 33-year-old veteran owns a career .296 average, including .330 against left-handed pitchers. He has also played for Tampa Bay and Kansas City in a big league career that started in 2003.
In 511 games for Atlanta from 2006-10, Diaz hit .305. He also collected 39 pinch hits and batted .285 as a pinch hitter.