The Belgian side, unbeaten in 15 home games in all competitions, missed out on the group stages last season when beaten by Liverpool in their third round qualifier.
However, their hopes have been hit by the injury to Defour - which will keep him out for the entire group stage.
He suffered a broken foot in Saturday's 3-0 win over Mechelen and has been ruled out for at least three months.
Liege's hopes will rest to a large extend on the shoulders of Serbia striker Milan Jovanovic, who admits Defour will be missed.
"Our captain is an important player," Jovanovic said on Standard's official website. "I really am sad for him and for the team team."
The Serbian believes Liege will be facing one of the best footballing sides in Europe when they face the Gunners.
"Our task will be very difficult," he said. "I believe that with Barcelona, Arsenal are the team who play the better football with very technical players.
"We have no experience in the Champions League. This will be a discovery for all of us. We will know quickly if we are at the level. But this will be very difficult.
"Arsenal are the big favourites in our group. Facing them, the solution will be to wait. Manchester City did it at the weekend.
"I think that we will have to do the same thing - wait and take advantage of the tiniest chance to attempt something."