The veteran goalkeeper was unexpectedly called into action in the 18th minute in Vicente Calderon on Tuesday after Petr Cech fell awkwardly in an aerial challenge and suffered a suspected dislocated shoulder.
|10/11||Chelsea are 10/11 with bet365 to reach the Champions League final|
On being called into action from the bench, the 41-year-old explained it was part and parcel of being a goalkeeper, telling ITV: "There's not much to think about. It happened so quick.
"It's a tough thing to do, but I had to go out there and try to do my job.
"Everyone was right up for it, which you would expect. We knew if we matched their determination our quality would come through. Okay, we didn't create many chances but it was a good performance."
Cesar Azpilicueta, who made a brave block from Diego Costa in the first half, echoed the sentiments of his Chelsea team-mate.
"It's a positive result," he said. "We knew it would be a difficult atmosphere.
"We fought hard and tried to keep the ball but the important thing is that we have the second leg at home.
"The players we will be missing are very important but we will fight to win."