Tabata has been a starter for the Pirates when healthy since his majors debut in June 2010. He signed a contract extension last August that will run through 2016 but struggled this season. Tabata was batting .230 with three home runs and 11 RBIs in 72 games.
Tabata also made some mental gaffes defensively and while baserunning, and manager Clint Hurdle hinted last month of a possible demotion. Tuesday, Hurdle said the team wanted to let Tabata play out of his slump in the minors.
Hernandez is in his second stint with the Pirates, having gone 2 for 10 with two RBIs in nine games from late May through early June.