Girard described his side's performance as "inefficient" after watching Mathieu Bodmer all but end their title aspirations with a goal in first-half stoppage time.
"We were inefficient, that sums up the way we played against a Nice side that appeared vulnerable," he told reporters.
"They didn't really put us in danger, but we conceded a goal at the worst moment.
"I don't seen any option for us but to work, the boys are already working well, but we must be vigilant and careful."
The result leave Lille in third place in Ligue 1, 13 points behind leaders Paris Saint-Germain.