Nico Williams struck for the 1-0 victory on Thursday night.
Valverde later said: "It's a valuable win because we rate our opponents highly. We knew it was going to be difficult," Valverde said in his pre-match press conference.
"We've played three different games and you have to sweat for every point in Europe. These are very good teams. We're happy with where we are in the standings."
On Slavia, he continued: "They're a great team. If they were in Europa League Pot 1, it's for a reason, and they have some incredible players.
"They've had the same coach for a long time, they play with a lot of energy and they press very hard from the first minute to the last. They never give up.
"I was expecting a game like this with a lot of back and forth. We won but with a lot of pain and suffering.
"Slavia played very well. It was difficult for us because they pressed individually. That has its complications and also its advantages as you can occasionally catch them close to the box. That's where the goal came from."