In the end the Ligue 1 side came from behind to win 3-2, but Gillot warned his players that they will need to be much better in the semi-finals to have a chance of reaching the final.
"We put ourselves in trouble by not starting the match well," Ligue1.com quotes Gillot as saying. "We played poorly in the first half and paid for it.
"We woke up in the second half and scored three goals before giving ourselves a fright in the last 10 minutes. We have the art of knowing how to make life difficult for ourselves.
"We'll have to play better in the semi-final if we want to qualify for the final. Lens's young side gave a lot. We didn't show much intent or determination but we're happy to have gone through."
Meanwhile, Lens coach Eric Sikora was pleased with the effort shown by his side.