Alonso told Sky Sports News: "It was a difficult decision to take from the referee, but we have seen in many previous games that it can be harder playing against 10 men. We produced our best minutes after the sending off.
"After that, United pushed really hard and it was difficult not to concede and our goalkeeper was fantastic.
"We knew it was a key week for us, we were fighting, it was make or break. It has gone fantastically."
The loss eliminated United from the competition.