Goals from Alharbi El Jadeyaoui and Benjamin Bourigeaud condemned Toulouse to a second successive Ligue 1 loss, and manager Casanova admits that it was a surprise to see his side produce such a disappointing display.
"I didn't expect to see my team start the match so poorly," he told reporters. "It wasn't at all what we wanted to do. We had planned to have a big first fifteen minutes by pressuring their experimental defence and rookie goalkeeper, but we didn't manage it and played terribly.
"Against a side who defended as well as Lens did, it was complicated to try and reduce this two-goal deficit. There are no easy matches in Ligue 1 and despite their injury absences, Lens put us under the pump. We didn't prepare well; perhaps we were complacent."
Toulouse will look to recover from the defeat in the Coupe de la Ligue clash with Bordeaux on Tuesday.