The current Manchester United shotstopper endured a nightmare start to the World Cup in Russia, letting a tame effort from Cristiano Ronaldo slip through his arms for Portugal's second goal in a 3-3 Group B draw.
He also faced criticism in Spain for a badly-positioned defensive wall when Ronaldo struck the late equaliser.
While the errors of De Gea's early Manchester United career are largely long-forgotten, he came into the World Cup having made a blunder to gift Switzerland an equaliser in Spain's penultimate warm-up friendly before the World Cup.
It should not, however, cloud Spain coach Fernando Hierro's judgement about his keeper unless the mistakes become more frequent, Bosnich told Reuters.
"No goalkeeper, no player, no human being is immune from mistakes, the important thing is that you get back up again," he said at a match between a travelling group of Australian fans and a local Samara team.
"And he makes the saves that no-one else can. If I've got a goalkeeper in my team and I am the manager and I know he makes saves that no other goalkeeper can, I'll accept the odd error.
"It's when it becomes a little bit more consistent that you start thinking to yourself, 'well hang on a minute'. You have got to weigh up risk over reward."
De Gea will have the chance to redeem himself when Spain meet Iran on Wednesday with the keeper's place in the team secure after Hierro gave him his public backing on Monday.
The process of rebuilding his confidence, however, will happen minute-by-minute, according to Bosnich who joined Manchester United from Aston Villa in 1999 and stayed at Old Trafford for two years before joining Chelsea.
"If he does play then he just needs to go through this next game, nothing flash, nothing incredible and just do his job, nice and simple," Bosnich said.
"Play the next game minute-by-minute, that's all you can do. If you start thinking about the past you are going to stay stuck in that moment.
"That's the only way you can do it. But if you start playing and you do a great save, but you think to yourself, 'what about the mistake I made last game?', you will never recover from that, you must keep going forward."
(Reporting by Toby Davis, editing by Ed Osmond)
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