Buendia struck, along with John McGinn, for the 2-0 win on Thursday night.
The Argentine later said: "We knew from the beginning that it was going to be tough. Feyenoord are really good – they showed that last season and, this season, they are top of their league.
"The atmosphere was really nice and they had a really strong first half. They created chances but we stayed strong and tried to play our football. In the second half we found a better rhythm with the ball and more solutions – and after we scored, I think we controlled the game."
Buendia added, "Feyenoord were brave; they were pressing very high, with a lot of intent. We tried to be confident with the ball and control their press. We were confident in our style, trying to beat them.
"We felt the support of our fans throughout. I don't know how many there were but we heard them during the game. They really help us through the season so we are happy for them when we win."