Mack was considered one of the Nittany Lions' best rising prospects after his one year in Happy Valley.
Clifford stepped down from his role effective January 2 to pursue other opportunities, according to a university release.
The Nittany Lions are 0-2 as underdogs this season.
The Penn State Alumni Association will host an official tour package for alumni and friends who are traveling to the national semifinals at the Orange Bowl, set for 7:30 p.m., Thursday, Jan. 9.
