Bellerin conceded Bayer were too good on the night.
He said, "In the first half I think we were solid, apart from a couple of chances they had. When we conceded the first goal we didn't come out with the right attitude in the second half. They are a good team with lots of quick, creative players and they play good football.
"It wasn't our best defensive display in the second half. We have a very talented squad but sometimes you don't play your best game. They played really well, credit to them, especially offensively. We want to enjoy this competition and get further.
"We have two games to play and anything can happen. We need to get to work."