Diaz will rest for the next week with lateral right-elbow discomfort and then be reevaluated, the Pittsburgh Tribune-Review reports.
The injury sounds as though it could be fairly serious. Diaz, who's batting .421 in 19 spring-training at-bats, hit .271/.330/.382 in 363 plate appearances for Triple-A Indy last year. He's currently ranked third among catchers on the depth chart. Ed Easley would likely serve as Pittsburgh's third option to start the season if Diaz is forced to miss time.