Johnson is facing up to 10 years in prison after being convicted this week of sexually assaulting a schoolgirl.
But Allardyce said he had no knowledge of incriminating documents handed to the club by police and was working under the impression Johnson would be pleading not guilty to all four charges.
“I was aware of his plea for all charges to be not guilty and on that basis, he trained and played for the team if and when I selected him,” said Allardyce.
“We were all at the football club stunned and shocked the day before the trial started when we heard a guilty plea come out.
“I was sat at home when it came on the news and I was just gobsmacked.”