The former champion runner, who won 1500m gold at the 1976 games, believed Willis would pull away from his competitors in the final stretch yesterday morning.
Instead, the 29-year-old faded badly in the last 300m of the race and eventually dropped to ninth place.
Walker said he was stunned to see Willis fail to push himself into contention after a strong build-up to the Games.
He expected him to deliver a medal and thought there was a strong chance of a gold.
"I thought he would move with 300 to go and he didn't move, he went backwards. I was just shocked because he's promised us a lot ... I was hoping he would get this one today because I thought this was his opportunity.
"I don't want to condemn the boy. But I was disappointed because I expected more."
A training issue or a problem with recovery could have limited Willis in the final, Walker said.
"At the end of the day, only he knows what went wrong. He's the one that's going to suffer most."
Walker said Willis should now wipe the slate clean and focus on the rest of his career.
Willis yesterday afternoon took to Twitter to explain what may have been behind his performance in the race.
"On reflection my poor performance wasn't likely a miss-timed peak, but inability to recover from rounds. How to fix that is the Q," he said.
He was thankful for the "overwhelming" support he had received.
"This Olympics has still been an amazing unique chapter in life."
