Girgensons has missed three games with an upper-body injury sustained in a 4-1 loss to Boston last week. He is expected to miss at least 10 days, with the Sabres getting a week off over the NHL's All-Star break next weekend.
O'Reilly had an assist during a two-game stint with the Sabres last month. He has four goals and 19 assists in 35 games with the American Hockey League Rochester Americans.
The Sabres host Detroit on Friday night.