O'Brien, a member of the British & Irish Lions this summer, has only played five games for Leinster this season following thumb surgery and an ankle problem, although Reddan was under no illusions how important he was.
But with others able to step up in his absence, including Jordi Murphy and Rhys Ruddock, Reddan remains confident they will adjust accordingly.
"It is very, very tough. Seanie will be ringing me, slagging me for mentioning other names," Reddan told the Irish Examiner.
"He is an exceptional player. I don't think it will be one guy. I think we will have to slightly adjust what we do a little bit in certain things.
"But, the back row played very well at the weekend. Seanie played very well. Jordi played very well and Rhys played very well.
"You've got Kevin [McLaughlin] to come back into that, Jenno [Shane Jennings], Dippy [Dominic Ryan]. You have an embarrassment of riches there."
Follow us on Facebook, join the conversation on Twitter, sign up to our YouTube channel for extensive match highlights and sign up to our newsletter for regular updates on the RaboDirect PRO12
63x46.png){kind=logo}