Slater started in 37 games over three seasons, including a streak of 30 consecutive starts for the Wildcats. He didn't allow any sacks this past season, while allowing one quarterback hit and five pressures in 11 games.
Slater's decision comes a day after Michigan offensive lineman Jalen Mayfield announced he, too, would declare for the NFL draft. With the Big Ten postponing the fall season, Slater decided it would be best to prepare for his future.
"The past few weeks have been a roller coaster of uncertainty brought on by the COVID-19 pandemic. This fall, I was excited to earn my degree and win a Big Ten championship with my team," Slater said in a statement. "Though I am still on track to graduate in December, it breaks my heart that I won't be able to strap up and battle with my brothers again in this time frame. While I can't control the events that led to this moment, I can control my response."
Slater could wind up as an early-round draft pick, as he was projected to be one of the best returning Big Ten offensive linemen for the 2020 season.
Northwestern offensive line coach Kurt Anderson described Slater as possessing special gifts, and someone with outstanding instincts.
Head coach Pat Fitzgerald also used the word "special" to describe Slater and wished him well in a statement announcing his lineman's decision.
"It's a rare player that can make an impact in the Big Ten in their first year of college, and even more rare to do it on the offensive line," Fitzgerald said. "Rashawn earned a starting spot almost immediately and never gave it up, helping pave the way to two bowl championships and a Big Ten West Division title. He has made the entire Northwestern community proud not only on gamedays, but in the classroom as an Academic All-Big Ten honoree, and we can't wait to see what his future holds on Sundays!"