Coach Mike Riley made the announcement Thursday at the completion of practice after Armstrong missed a fourth consecutive workout with his right foot in a boot. He aggravated a foot injury Saturday in a 30-28 loss to Northwestern.
Armstrong regressed in his rehabilitation late this week, Riley said, but he does not think the injury will be a long-term issue.
The first-year coach said he feels confidence in Fyfe before his starting debut.
"He is not held back," Riley said. "He has run all these plays. He was the sole guy this week to take the first-string reps. So the knowledge of everything about what we're doing, he's familiar with.
"I'm excited for him, and I think he'll do very well."
Armstrong had started 22 consecutive games for Nebraska (3-5, 1-3 Big Ten), which previously had never lost more than five games before Nov. 1.
The 6-foot-2, 210-pound Fyfe came to Lincoln as a walk-on out of Grand Island, Nebraska, in 2012. He completed five of seven passes for 33 yards and one touchdown Sept. 12 in the Huskers' victory over South Alabama. Last season, Fyfe played in seven games and completed four throws for 40 yards.
Armstrong has thrown for 2,052 yards and is 152-of-284 passing this season. His QBR figure of 76.0 ranks 22nd nationally and second to Michigan State's Connor Cook in the Big Ten. Armstrong also is the Huskers' second-leading rusher with 257 yards.