The Spanish shot stopper was replaced by Anders Lindegaard between the sticks as the Red Devils narrowly defeated Nigel Adkins' men 3-2 at St. Mary's Stadium.
And the Scot confirmed after the game that De Gea's spill against the Cottagers, which allowed them to get within a goal of earning a point at Old Trafford, cost the former Atletico Madrid man.
|DE GEA THINGS GO|
|2/1||Manchester United are 2/1 with William Hill to win the Premier League|
"With the form he's been in, he's been making some fantastic saves, but one error like that could have cost us the game.
"It's just a learning process for him and he'll be back in a couple of weeks' time after the international break.
"Anders had his first game. I thought he was a little bit nervous at points but he's a very good keeper also. I don't think it's a big issue but it's something you have to deal with."