The veteran scrum-half spent four seasons with the Premiership side, winning a European and domestic title in that time.
Despite a tough few years since then, a new home, a new team and an increasing fan base have Wasps in great shape.
They lie fourth in the Premiership currently, the same position Leinster find themselves in the GUINNESS PRO12, while a win away to Harlequins last time out took them within 80 minutes of a European quarter-final.
That win at the Stoop, coupled with Leinster's defeat of Castres, has made for a dramatic finale to Pool 2 with the winner of Saturday's final fixture guaranteed a place in the last eight.
And having always kept a watch out for his former employer, Reddan believes his side will have to be at their most clinical to ensure Wasps' resolute defence is breached.
"Their resurgence has been pretty amazing, in some ways. They are fourth in the Premiership now," Reddan told the Irish Times.
"They have scouted pretty well and they have got a lot of guys who you wouldn't have heard of before but who are now delivering big games. It's not like they signed Bryan Habana.
"They signed guys like (Ashley) Johnson and (Nathan) Hughes, who all of a sudden are big players, and that's a sign of a good club.
"I think from watching them play and the intensity they play with, their defence is still very strong as you could see from the weekend. I wouldn't be comparing them to Wasps of old.
"I think I'm probably more impressed with the turnaround they've had as a club. It's so hard to change your culture, never mind the whole roster as well, and still come out with a team that's consistently doing well."
With Europe offering the Irish region a chance to build some momentum for when the GUINNESS PRO12 returns, it was perhaps fitting that there were signs of improvement against Castres.
But Reddan admits they still have a lot of work to do if they are to achieve their goals this season.
"We got a few things right at the breakdown that allowed us to play a bit more," he says. "You take any breakdown, there's always a reason why the ball is not quick.
"The angle you carry in at, the height you're at, how hard you fight. Is the barrel there quickly, is the other barrel there quickly? Does the man get in? Does the man get the ball away? There's probably ten jobs to do."
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