The Roughnecks officially named Walker their starter Wednesday, three days before their season opener against the Los Angeles Wildcats.
The 5-foot-11 Walker played under current Carolina Panthers coach Matt Rhule at Temple and led the Owls to back-to-back 10-win seasons in 2015 and 2016.
Walker is Temple's all-time leader in virtually every passing statistic, including yards (10,668), touchdowns (74) and completions (830).
"I'm an undersized QB so I'm always going to carry a chip on my shoulder," Walker told the Roughnecks' official website. "I am always going to walk on the field like I am the best player on the field."
Walker won a preseason competition with Cook, a fourth-round NFL draft pick in 2016 who started under center for the Raiders in a postseason game.