Marseille, who started the defence of their title with a 2-1 home defeat by promoted Caen, conceded three goals in 15 minutes in the second half before reducing the arrears with late goals by Nigeria defender Taye Taiwo from the penalty spot and Ghana forward Andre Ayew.
There was more bad news for Marseille fans after the match with confirmation that Senegal striker Mamadou Niang, last season's top Ligue 1 scorer with 18 goals, was joining Turkish side Fenerbahce.
Midfielder Gael Danic had put northerners Valenciennes ahead with a volley from inside the box and forward Gregory Pujol increased the home side's lead with two goals in quick succession.
"The whole Mamadou (Niang) affair has had negative consequences for the entire group," Deschamps, who had made clear he wanted Niang to stay, told reporters. "Now we need positive news to reverse the trend."
"I really didn't believe we would have so much turbulence in the close season but we need to face it," Deschamps said.
"I really don't know what happened in that second half," Ayew told reporters. "What I do know is we made mistakes we must not do again. We must bounce back and must do it straight away. We have no time to lose."
The season's second round of matches featured the two first Ligue 1 games played on synthetic pitches, at Nancy and Lorient, the two stadiums leading the experiment in France.
