Argentina striker Lopez, who signed from Porto during the close season, has played two Ligue 1 games and both legs of a Champions League playoff, scoring five goals and setting up several more.
Lopez has proved a suitable replacement for France striker Benzema, who scored 37 league goals for the side over the past two seasons before leaving for Real Madrid.
"He's intelligent, he knows how to adapt to our playing system and he has a very interesting range," Lyon coach Claude Puel said.
"He's not only a useful scorer but he's also very good at setting up others. He's a complete football player."
Lopez was involved in four of Lyon's goals in a 5-1 home demolition of the Belgians, then scored a hat-trick in their 3-1win in the return leg on Tuesday.
Gourcuff, whose side will try to extend their record of consecutive Ligue 1 victories to 15, recognised Marseille's strength and said their coach Didier Deschamps knew all about football at the highest level.
"It should be a great match," Gourcuff told French television channel Canal Plus.
"We'll take it very seriously. We know our run of wins will have to end one day but we want to make it last for as long as we can."
