After failing to qualify for the tournament in the 2015-16 campaign, the Red Devils had to settle for the Europa League.
The club were on course to suffer the same fate last season when they finished sixth in the Premier League, but they were guaranteed a spot in the elite competition's group stage after winning the Europa final.
"It was hard to watch Champions League games on TV. For a club like Manchester United not to be in those games it was hard," The Mirror quotes De Gea as saying.
"But now we're back and we're motivated to win Champions League games and to bring the Champions League back to Old Trafford for the fans. To do that, we need to play our best football in these games.
"It's good to be back in the Champions League, especially at Old Trafford. It's almost two years without a Champions League game at home. We have a high level of motivation and we want to show that on the pitch.
"It's another step for us to play at the highest level, in the Champions League with the best teams in Europe. To win the Champions League, we know, is really difficult, but I think our team is now is really good."
United will kick off their European campaign with a group match against Basel at Old Trafford on Tuesday night.