The Pirates announced on Monday that second baseman Neil Walker had been placed on the disabled list after undergoing an appendectomy. To replace Walker on the active roster, the Pirates say they'll call up outfield prospect Gregory Polanco.
The 22-year-old Polanco is widely regarded as one of the top prospects in all of baseball. Entering the 2014 season, he was ranked as the no. 10 overall prospect by Baseball America, and his current line of .347/.405/.540 at Triple-A (15 stolen bases) has only improved his standing.
While Polanco has been deserving of a call-up for some time, the Pirates were almost certainly waiting until the "super two" deadline for accelerated arbitration-eligibility passed, which it likely has. He'll be up in time to debut against the Cubs in Pittsburgh on Tuesday.
As for the veteran Walker, he's batting a very productive .280/.353/.452 on the year with 11 homers in 62 games.