The 28-year-old midfielder, who was sacked by the club after admitting charges of engaging in sexual activity with a 15-year-old girl and grooming on the eve of his court appearance last month, was convicted on a further count of sexual activity with a child and acquitted of another on Wednesday.
He now faces a lengthy spell in jail as pressure mounts on the Black Cats to explain what they knew and when amid claims made in court that they continued to play him despite knowing the detail of the allegations, something they have strenuously denied.
Manager Allardyce, who was appointed in October, some seven months after Johnson's arrest, admitted he was "gobsmacked" when he learned of the guilty pleas.
He said: "I am hugely disappointed on Adam Johnson and what has happened, but my sympathies don't lie with him, they lie with the victim and the family.
"Adam, as far as I know, was pleading not guilty all the way through from the day I arrived - obviously, this incident had happened well before I got here.
"On the basis of that information to me that he was pleading not guilty on all charges is why we continued to let him train and play for us.
"We are 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 just was gobsmacked because as far as we were concerned, it was not guilty on all charges.
"Now the judgement has been made, we all feel extremely let down by what has happened and by what Adam has done, and certainly feel a lot of sympathy for the victim and the family.
"That has now hopefully given them some peace and they can now get on with their lives and we can get on with our football."
In the meantime, Allardyce revealed he has spoken to the remainder of the squad about the situation and warned them not to lose focus as they attempt to stave off relegation from the Barclays Premier League.
He said: "Everybody is well aware of the situation that it has left the club in and left them in
I think they are all as disappointed as we are as a football club
They are all going to have to make sure they put that behind them."
Allardyce's comments came as the club found itself at the centre of criticism for suspending Johnson for a fortnight after his arrest rather that throughout the police investigation into his crimes amid suggestions - which they insist are inaccurate - that they played him because they needed him on the pitch.
A spokesman for the NSPCC said: "If they had known he had kissed a girl prior to is guilty plea, then we think they should have suspended him pending the ongoing investigation
It would have sent the right message to people that this is a serious offence and needed to be properly investigated."
There were strong words too from Clare Phillipson, director of charity Wearside Women in Need, who said: "If that's true, then at that point they definitely should have suspended him because in allowing him to continue to play, tens of thousands of fans and lots of other people thought, 'Oh well, the club is still letting him play, the case against him can't be very good, he probably is innocent'.
"Once again, that leaves the victim feeling vilified and not believed and feeling that she was at fault and it was her who was to blame."
Source : PA
Source: PA