Benfica shocked Real Madrid 4-2, with both teams now into the hat for the round of 16 play-offs.
Bellingham said afterwards: "Obviously, it's horrible to lose like this. We didn't expect to have this kind of game and we have to look at it again. The goals we conceded, the things that happened. I don't have much more to say. I'm not sure what to think, but it doesn't look good.
"One of the things that happened is that they were more intense and aggressive. They've won all the 50/50 balls. The basic things that we usually do well, like generating out of nothing with our talent, we weren't able to deliver in this game."
Meanwhile, Real teammate Kylian Mbappe also said: "In football, games start on 0 minutes and not 45 minutes, so in the end the defeat is deserved.
"I can't say it's not deserved because we didn't play well to win this game. We didn't play well and they did. Benfica at home is always difficult. We knew it before and I think it was our fault."