[5][6][7] On January 11, 2018, Harrison announced his decision to forgo his remaining eligibility and enter the 2018 NFL draft.

Harrison received interest and was regarded as a top prospect at his position due to his combination of size and athleticism.

He attended visit and private workouts with multiple teams, including the New York Giants, Pittsburgh Steelers, Detroit Lions, Tennessee Titans, Jacksonville Jaguars, Minnesota Vikings, New England Patriots, Cincinnati Bengals, Carolina Panthers, Seattle Seahawks, Houston Texans, and Philadelphia Eagles.

[9] At the conclusion of the pre-draft process, Harrison was projected to be a second round pick by NFL draft experts and scouts.

[17] Harrison entered his rookie season slated as the backup strong safety behind Barry Church.

[21] In a Week 7 win against the Bengals, Harrison recovered a fumble forced by teammate DJ Hayden and recorded an interception.

[23] In Week 5 against the Indianapolis Colts, Harrison intercepted a pass thrown by Philip Rivers and returned it for a 47-yard touchdown during the 32–23 win.

Two weeks later, during the loss to the Bengals, he intercepted a pass by Jake Browning and returned it 36 yards for a touchdown.