Pena sustained a fracture in his left fibula during Double-A Biloxi's game against Birmingham on Saturday, Curt Hogg of the Milwaukee Journal Sentinel reports.
Pena suffered the injury on a slide into third base and was carted off the field. The 19-year-old required a splint and the use of crutches to get around, but he's expected to begin his rehab program next week. The fracture continues a tough run of injury luck for Pena, who missed a month earlier this season after collapsing in the dugout due to overheating. He was promoted to Biloxi in mid-August and has a .268/.328/.339 slash line in his first 16 contests at Double-A.