Lewandowski struck in the second-half as Barca won 1-0.
The Pole said afterwards: "Before the match we knew we had to win. We are a few points behind and the most important thing was to win. Right now we have to play to win and then look at the standings.
"A game like today is very important. We can't always win by 3, 4 or 5 goals. Sometimes a team comes along that plays very poorly and doesn't want to play, but that's football. We played with patience and vigilance and in the end we scored one more goal than our opponent. We are very happy.
"There are games where we don't have many scoring opportunities, but in the end we get into the box and we look for a pass and we could have scored a goal. We have lost a lot of points in the league. Now we have to win."