Zwinak injured his lower left leg on the opening kickoff against Ohio State on Saturday and was carted off the field.
A Penn State spokesman says Keiser was hurt in practice last Thursday.
Zwinak has rushed for 2,108 career yards with 11 100-yard games to place 15th on Penn State's career rushing list.
Keiser has made 11 career starts and is third on the team with 25 tackles this season.
Both players already have graduated from Penn State.