Fournier, whose side remain just one point above the relegation zone, believes that Aissa Mandi's second-half red card effectively ended the game as a contest.
However, he said that Claude Puel's men fully deserved to win.
"We stumbled on a better team," Fournier told reporters. "They were better equipped technically, which took more commitment.
"To compete with this kind of team you must be 100% tonight and we were not. After the red card it became impossible task against a team who fully deserved their victory.
"We wasted too many of our attacks, which prevented us from putting [David] Ospina under pressure."
Reims welcome Paris Saint-Germain to the Stade Auguste Delaune next Saturday.