De Gea was partially responsible for Jay Rodriguez's early goal, part of another jittery performance during which he also nearly spilled a Rickie Lambert free-kick.
"It is clear to me that any free-kick near to the box, or a corner, presents a goalscoring opportunity," he said.
"That doesn't happen as much in the Spanish league.
"Also, the goalkeeper in Spain is a bit more protected than here. That makes their job here a lot harder."
At least for the Saints' goal, De Gea has been absolved of blame by Michael Carrick, whose weak backpass put the United 'keeper in trouble.
"Michael's pass back was a bit soft and Michael held his hand up - it's great to see that, honest players like Michael saying, 'Sorry boss, sorry lads'. It's great," said Ferguson.
That De Gea's woes did not prove fatal was thanks to Wayne Rooney's brace and a backs-to-the-wall defensive effort in the second period.
Although Robin van Persie had a header saved by Artur Boruc and what appeared to be a legal goal disallowed for offside, it was the Saints who carried by far the most threat.
"In the second half Southampton produced the best performance anyone has done here this season," said Ferguson.
"But my experience of this club, when you are going for championships, is that there are always games where you can say we were a bit lucky. That was one of those nights."
Pochettino agreed that his side could count themselves unfortunate, having tried to exploit some nervousness in the home ranks.
"We did notice that," he said. "In the second half we were dominating the game and taking the game to them.
"We should have scored at least one. But, especially going forward, the way we played in the second half was the way we want to continue playing."