The Gers suffered a 3-0 friendly defeat by Bochum on Saturday.
With Madjid Bougherra left out as a precaution over a tight hamstring, Rangers lined up with the out-of-contract David Weir and Kirk Broadfoot, who has not played a competitive game since November, in central defence.
But they conceded three second half goals against Bochum to make it two defeats out of two on their German tour.
Rangers have been trying for several weeks to secure a deal that would bring back Cuellar from Aston Villa, where the Spanish defender left for in 2008 following a successful solitary season at Ibrox, helping the club reach the UEFA Cup final.
Manager Ally McCoist has admitted the trail has gone slightly cold but his left-back shares his enthusiasm in bringing back the former Osasuna player.
Papac told the club's official website: "I know that the manager is interested in bringing Carlos Cuellar back to the club and that would be great.
"He is a very good player. We know how he can play after his great season with us before."
Rangers, who face Bayer Leverkusen on Wednesday, could have done with Cuellar in Bochum but Papac believes they will soon iron out the errors that led to their late collapse.
Papac said: "We have only been in pre-season training for two weeks and I think there were some heavy legs in the second half against Bochum.
"We will become stronger in the next two weeks because we need to be ready for the start of the season.
"I think we can be ready but I feel we need some more players because we don't have a very big squad."
Steven Naismith also refused to panic but he admitted they cannot afford to concede goals like they did against the German second tier club.
"Obviously the game was a friendly and nothing is won or lost at this stage of the season," the Scotland player said.
"More than anything it's about building up fitness and getting match practice, albeit against teams that might be further ahead in doing that than you.
"All the same, we were disappointed with the standard of goals we lost. It was terrible and we need to be doing better than that.
"It's up to us to cut that out before the start of the season and I'm sure we will.
"There weren't many positives to take from the game and we need to sharpen up, but we'll do that."