In short, the answer is because De Gea will be a world class goalkeeper for a very long time. But first he must embark on a steep learning curve before reaching that level.
Like every goalkeeper, De Gea has made errors but two high-profile mistakes in as many weeks have given reason for fans to call for Anders Lindegaard to be the man between the sticks as they seek confidence heading into an eight game run in which Manchester United face Tottenham, Arsenal, Manchester City, Chelsea and Liverpool.
The good news for De Gea was that neither mistake has proved too costly.
His late effort to save Edin Dzeko's 30 yard drive at Wembley was forgiven as his team-mates turned the game around to win 3-2 and at the Hawthorns on Sunday afternoon the trick was repeated as United snatched a victory when a draw appeared to be the most likely outcome. All of this following on from De Gea allowing Shane Long's effort to go underneath his desperate dive when the save should have been routine for a goalkeeper of his quality.
Early signs of the Premier League season, despite there only being one round of matches, suggest that dropping points will be punished even with the campaign only being in its infancy.
If De Gea continues in this vein then it seems inevitable that Sir Alex Ferguson will opt for Lindegaard in goal as the risk of dropping points is too great if the young Spaniard starts such games.

Although his distribution is excellent, De Gea is let down by his inability to command the area. This problem may come from the fact that his English is poor but the defence do not seem to have anywhere near as much confidence in him as they do in Lindegaard.
Lindegaard's career at United consists of two FA Cup appearances but after enjoying a positive pre-season in America, the 27 year-old seems determined to make the most of any opportunity that he may be presented with.
It may well be an opportunity that is provided by De Gea's inexperience and vulnerability.
It is now a well-known fact that he conceded the most goals from outside the box in La Liga last season and Dzeko's goal in the Community Shield did little to dispel the concerns over his capability in dealing with long-range efforts.
De Gea will, inevitably, be peppered with shots from distance by the opposition and, such is the unforgiving nature of the Premier League, if one of them does find a way past the Spaniard then question marks will again be raised over a so-called 'dodgy keeper'.

De Gea's mental strength is being intensely tested early on in his Old Trafford career as well as his ability and we will find out sooner rather than later whether or not he is up to the challenge of filling the rather large gloves left by the retirement of Edwin Van Der Sar.