Shilton watched De Gea's early performances following his £18 million move from Atletico Madrid last summer through splayed fingers.
As the mistakes mounted, it became increasingly obvious De Gea was not up to the task of filling the vacancy created by Edwin van der Sar's retirement.
Yet Shilton was impressed at the way De Gea battled back.
However, after conceding four times at Old Trafford last weekend as United tossed away two vital points in the Barclays Premier League title race, the former England keeper feels De Gea has to answer some pretty significant questions.
"From a technique point of view, I wasn't totally convinced when I saw him in the early part of the season," said Shilton.
"He looked very raw, he didn't quite look like a dominating keeper and not what you would expect from Manchester United.
"There have been other keepers who have found it hard to adjust to playing for Manchester United, Fabien Barthez for example, but he has recovered well in the last few months.
"However, this derby is the acid test now.
"It is possibly the biggest game of his career, and if he can come through it and have a good game, it could clinch the title for United and cement his progress.
"But if he has a wobbly game and United get beaten, people will still be pointing fingers."
Shilton does feel De Gea still has some work to do to prove himself.
However, he believes the key will be for the 21-year-old not to let his self-belief dip, no matter how much of a struggle it might be at times.
"He's still got a way to go to justify being at United but he is only a young lad," said Shilton.
"It's all about confidence and self belief.
"You've got to focus on doing your job properly, forget about winning the championship and not allow the pressure to get you.
"That is the only way you can play sport. You can't think about what is on the game."
With Anders Lindegaard ruled out for so long with an ankle injury, De Gea is at least certain of his starting berth, which must make things easier.
While trying to second guess Ferguson's starting line-up is notoriously unpredictable, there are obvious selection dilemmas, even without taking into account the injury Jonny Evans suffered against Everton that prevented him from training earlier this week.
Once again, Rafael struggled defensively against the Toffeemen, suggesting Ferguson may elect for either Phil Jones or Chris Smalling to fill the right-back berth.
And the United boss must be giving some thought to trying to block the midfield channels, which would mean a recall for either Ryan Giggs, Tom Cleverley or Park Ji-sung at the expense of Nani, Antonio Valencia or Danny Welbeck, who has struck up such a productive striking partnership with Wayne Rooney.
Either way, as Ferguson mulls over his final choices at United's weekend base in south Wales, he knows actually informing players they are not going to be involved is not easy, even after 26 years at the Old Trafford helm.
"In big game situations, all the players want to play," said Ferguson.
"That makes it very difficult because I am going to be leaving three or four players out of the squad altogether.
"It is always hard for me. It doesn't matter how much experience you have, or how long you have been in the game, having to leave good players out of big games is tortuous."