The 33-year-old lock fractured his arm during the 23-21 triumph in the opening Test against the Wallabies on Saturday, and has now been ruled out of the rest of the series.
O'Brien insists that the absence of his compatriot will have a negative impact off the field as well as on it.
"I think just his presence," O'Brien told reporters.
"He has a lot of experience, he's a big physical man, he has won lots of trophies in his career and he has been playing at the top of his game for so long now.
"But off the field as well he is someone the younger fellas do look up to. You can confide in him and talk to him and he will be missed all round."
The Lions face the Melbourne Rebels at 10.40am on Tuesday.