Emmanuel Herrera's 42nd-minute goal looked to have given the reigning champions their first win of the season, until Alain Traore and Jeremie Aliadiere struck in the dying seconds to record a memorable victory.
Jeunechamp said that there will be a target on La Pallaide's back after last season's exploits, and the team must rise to meet that challenge every week.
"When you are a goal up away from home after having drawn at home, you can't get carried away," said Jeunechamp. "Those last three minutes, it is going to be very tough this season, we know that, wherever we go, teams will be waiting for us.
"We must react and use what happened here as a lesson. We will be able to see, from the tough moments like these, if we are as solid a side as last year. But hopefully we can use what happened to our advantage and put things back in order.
"We need to first of all make sure we get to 42 or 43 points - if some of the boys haven't realised that then they will be reminded of it every time they step out on to the pitch on a Saturday."
Montpellier welcome Marseille to the Stade de la Mosson next Sunday.