Roman arrived in the summer on a three-year deal from Rapid Bucharest and has impressed with three goals during pre-season.
But the Romanian international winger has been ruled out for around six-months having damaged his Achilles tendon in training.
"We are a bit under shock. It is a big, bad blow because we lost a player we were really relying on," said Casnova.
"He already showed a lot of qualities."
Toulouse could try to sign a new right winger soon, and chairman Olivier Sadran said: "A new reinforcement? Indeed, that injury will make us think about it."