But Ospreys forwards coach Gibbes says the region has been boosted by the return of Wales stars Justin Tipuric, Dan Biggar and Rhys Webb, who are all set to feature at Principality Stadium on Sunday.
"It's pretty simple - we have to replace Alun," said Gibbes. "We prepare the boys all the way through the season and they've got to step up and fill that gap.
"It's hard to replace a guy like Al, but we have got to do it. We've got class players coming back and I think they're going to have a massive impact for us.
"They've got to play their part and try not to carry the team - but it's exciting to have them back with us."
Ospreys suffered a shock defeat to Pro12 strugglers Treviso in Italy last weekend. But the Welsh region blitzed through their Challenge Cup pool, finishing 14 points clear of Lyon with a maximum of 30 points.
Gibbes now wants Ospreys to be mentally prepared for Stade Francais, who like Treviso, are fighting near the bottom of their domestic league.
"What we learned against Treviso is that you can train the house down during the week, but if you don't mentally turn up to play, you get exposed," he added.
"Mental preparation is key. We've taken a bit of a kicking and that's hurt us. We've got to make sure we learn from that and see what happens next.
"The boys have got that challenge ahead of them this weekend."
Stade arrive in the Welsh capital on the back of a morale boosting Top14 victory over European Rugby Champions Cup quarter-finalists Toulon.
Their head coach Gonzalo Quesada said: "We have already been preparing for this quarter-final for a long time, we go to Cardiff to play a big game in the European Challenge Cup.
"We can have fun trying to get to the head of the Top 14 towards the end of his season and we go to play this quarter-final in Cardiff in a good spirit.
"Against the Ospreys, it's going to be a big game, but we can afford to go there knowing that in the Top14, nothing is finished. Far from it."