The Swiss side threw the tie wide open just before half-time, courtesy of a clinical strike from Mohamed Salah.
Look back at an efficient display from Chelsea with Sports Mole's minute-by-minute commentary below.
BASEL SUBS: Vailati, Park, Ajeti, David Degen, Diaz, Cabral, Zoua
The Spanish manager was keen to stress the importance of taking Basel seriously tonight, saying in his pre-match press conference: "Winning a trophy is important to everyone. We are not in the final yet and we are playing against a tough team. I have to concentrate on my business, that is to beat Basel."
"Basel will come trying to score, they did it against Spurs and I am sure they will have the same approach. They need to score.
"We have a good team and we are so close to the final we have to be concentrated. I have a lot of confidence in my players."
He told reporters: "It's going to be a dangerous game. Even though we have two goals away, a great result, it's that kind of game that can be very risky.
"If we don't go into the game thinking about winning, thinking about defending well, thinking about trying to win again, it's going to be difficult. We still know we have to fight if we're going to be in the final.
"We got a great result in Switzerland, but now we have to play another 90 minutes and I'm sure they're not going to give up. They're a team who try until the end. It will be tough, for sure."
He said: "Chelsea are strong in defence and attack, we know that, but my team also have very good qualities, we showed what we are capable of against Tottenham.
"We will be brave, we'll play with confidence and, who knows, maybe we can achieve a small miracle."