Jelen has scored four goals from eight games so far and he was voted the league's player of the month in October. He injured his right thigh during training on Wednesday and Auxerre fear a repeat of their bad start to the season when they had to make do without the 28-year-old Pole.
"When he plays, everything is much easier for everybody. He is the one who gives the depth we need in our game. Without him, we will face the same difficulties we had during the summer," Auxerre coach Jean Fernandez told reporters.
The Burgundy side were bottom of the table after they lost the first three games of the season. Since Jelen returned from injury in early September, Auxerre have been undefeated in the league and they went top of the table with a seventh consecutive win last weekend.
They have 26 points from 13 games, one more than Girondins Bordeaux and Olympique Lyon, third on goal difference. Paris St Germain are 13th on 16 points, with one game in hand, after winning only one of their last eight league games.
Bordeaux visit mid-table Nancy on Sunday after being defeated four times in their last six league games.
The French champions said they would now focus on the Ligue 1 title race after securing top spot in Champions League Group A with a convincing 2-0 win against Juventus on Wednesday.
"We are now going to turn our minds fully to Ligue 1," Bordeaux coach Laurent Blanc, who helped Auxerre to win their only French title as a player in 1996, told reporters.
Playmaker Yoann Gourcuff is still doubtful for Sunday's game because of a sore groin.
Olympique Lyon aim for their first win in almost a month when they host Stade Rennes on Sunday while Olympique Marseille, seventh on 22 points with one game in hand, visit Lens on Saturday.
