City are believed to have paid more than £50m to sign Walker from Tottenham Hotspur, with the right-back penning a five-year contract at the Etihad Stadium after completing a medical.
Lineker, however, has revealed his surprise at Walker becoming the most expensive defender in the world.
Fullback, Kyle Walker becomes the world's most expensive defender at £50m plus. Imagine how much he would cost if he could cross the ball.— Gary Lineker (@GaryLineker) July 14, 2017
Walker, who is a 27-time England international, leaves Spurs having made 228 appearances for the London club since debuting during the 2009-10 campaign.