Manchester United settled for a disappointing draw with their top-four rivals on a night where the weather and a lack of quality combined to produce an ugly display.
The hosts took the lead before halftime on a goal from Scott McTominay, putting an exclamation point on an otherwise drab first half, before Pierre Emerick-Aubameyang produced the equaliser in the second half.
The draw was not without a bit of controversy, though, as Aubameyang's equaliser came following a VAR review for offside.
While the forward was clearly onside, the linesman had raised his flag, prompting the Red Devils' defence to ease off in the seconds prior to the goal.
De Gea says that the mistake is one United must learn from as players continue to adjust to the use of VAR in the Premier League.
"Of course [we were distracted]. If the linesman keeps the flag down... but we also know that there's VAR as well, so if it's offside, it's offside," he told Sky Sports.
"There's nothing else to say, we just need to keep focused on our game. We need to learn from that mistake.
"It was a big mistake for us. We gave away an important goal.
"I think the game was well balanced so they had good chances and we had good chances. But we're playing at Old Trafford so we need to win these games if we want to finish in the top four."
With the draw, Ole Gunner Solskjaer's side are off to their worst-ever Premier League start, and De Gea says that the team must find a way to reverse that by picking up three points in matches like the one they were in on Monday.
"This is Manchester United. We're playing at Old Trafford so we have to win these games," De Gea said
"We have a young team but the lads have to react and play better than they are. We are a team. We know we have to improve and we are all working really hard to do it.
"We had a lot of good chances – of course, they did too, but I think we had the better ones.
"We have to keep learning. We have a young team, so it's time to keep looking forward and try to win the next game."