The midfielder came off the bench inside the final five minutes before adding the French side's third goal in their 3-1 win.
The Argentine told reporters: "It is a very important goal. A two-goal cushion is very good before the second leg. I am happy I helped the team win this game. Our team is very united and compact. And it is a very important thing. I enter then I score, I am lucky. I hope to continue."
The second leg of the quarter-final takes place at Stamford Bridge next Tuesday.