In his first career game, Walker returned an interception for a touchdown against Penn State that capped a Northwestern win.

For his performance that season, Walker received All-American recognition from the Associated Press and Sports Illustrated as well as Consensus First-team All-Big Ten Conference honors.

[9] Upon entering training camp, Walker competed to be a starting inside linebacker against Sean Spence, Antonio Morrison, Jon Bostic, and Edwin Jackson.

[10] Head coach Chuck Pagano named Walker a backup inside linebacker to begin the regular season, behind Bostic and Morrison.

[11] He made his professional regular season debut in the Colts' season-opener at the Los Angeles Rams, but exited in the second quarter of their 46–9 loss after sustaining a hamstring injury.

[20] On September 23, Walker recorded nine combined tackles and made his first career interception off a pass by quarterback Carson Wentz during a 20–16 loss at the Eagles in Week 2.

[21] In Week 13 against the Tennessee Titans, Walker recorded a team-high 12 tackles and forced a fumble on running back Derrick Henry on the first play of the game which was recovered by teammate Malik Hooker in the 31–17 loss.

[25] In Week 13 against the Houston Texans, Walker recovered a fumble lost by Deshaun Watson late in the fourth quarter to secure a 26–20 win for the Colts.