United go into the match with seven points from their three Group A outings, and requiring just one more win to reach the 10 Sir Alex Ferguson always felt was enough to secure a knockout berth.
Given their opponents have yet to muster so much as a draw on their return to the elite competition after a 10-year absence, the task appears straightforward.
Moyes and De Gea are united in their belief looks can be deceptive.
"I certainly won't take anything for granted," said Moyes.
"I have a lot of respect for Sociedad, and we will have to play well on Tuesday if we are going to get those 10 points which will get us over the line."
De Gea added: "It's very difficult to play at the level Real Madrid and Barcelona are at.
"They might not be at the top level yet but it's obvious Real Sociedad have been playing great football over the years."
United's task has been made slightly more tricky by the absence of Jonny Evans, Rafael and Tom Cleverley.
The trio had to be replaced at half-time during the weekend win at Fulham with back and ankle injuries and double vision respectively.
And their absence from Monday's flight places a question mark over their availability for Sunday's crucial Premier League encounter with leaders Arsenal.
Not that Moyes is giving much away on that front.
"We don't have a lot of injuries," he said.
"It's probably the best Manchester United have had for a long time."
United cannot actually qualify with two games remaining.
But it is remarkable how smooth their progress has been compared to their early-season troubles in domestic combat.
That form does appear to have turned. Yet Moyes has little in the way of explanation for it.
"There is nothing which we have done differently," he said.
"Everything we're doing in this Champions League we're doing in the Premier League as well.
"The players are used to it. They are experienced at this level and that helps.
"I hope that continues."