A clearly stunned Diaw went down and needed treatment before being stretchered off, and the game was halted before being abandoned following a crisis meeting.
"The doctor who examined the Clermont goalkeeper certified that he was not in a condition to return to the field. The match is therefore halted definitively," referee Florent Batta told media at the Stade de la Mosson.
Montpellier were leading 4-2 and on course to win their first home game of the season until the incident, and now risk serious punishment from the French footballing authorities with the prospect of having to replay the game without their fans at the very least.
A police spokesman told AFP that the suspected perpetrator had been arrested and taken into custody.
Diaw was able to return to Clermont with his teammates after the game and was due to undergo further tests on Monday, the club said.
"We are disgusted, disappointed and sad," admitted the Montpellier president, Laurent Nicollin, as he complained that the actions of one individual "could penalize the whole team."
