Pienaar, who began his career as a scrum-half and has also played international rugby on the wing, started the first two Tests against the Lions in the fly-half shirt earlier this summer.
But with the impressive form of Morne Steyn arguably forcing his hand, Springbok head coach Peter de Villiers opted to play Pienaar at full back for last Saturday's win over Australia in Perth.
"I think Peter's decision was a good one," Joubert told South Africa's Business Day.
"Ruan's such a talented player, so he can fit in anywhere. With his vision, he will fill in at full-back really well.
"I think he's done all the right things a full-back has to do. Overall, his was a very good performance."
And Joubert, who was a World Cup winner back in 1995 before featuring against the Lions two years later, has no doubt that Pienaar can make the position his own for the foreseeable future.
With Frans Steyn - the man who faced this year's crop of Lions in the No15 shirt - soon heading off to play his rugby with Racing Metro in France, and previous incumbent Conrad Jantjes still recovering from a broken leg, Joubert believes Pienaar will become a regular as the last line of defence.
"Frans is young and has been solid at the back this season, but he will be going to France soon. I think Ruan will be able to hold his own in his absence," added Joubert.
"What I liked was that he was joining the line quite a bit and gave the Springboks more of an attacking dimension. So I think it's a good thing they kept him there.
"He's still finding out that he should not only be joining the back line between the centres and wing, but become a secondary fly-half. With more experience, he will start to work that out. He also provides extra attacking options on the blindside.
"I also think the way Peter's looking at it, Ruan will provide another goal-kicking option, and that's also important. Morne Steyn can have an off day or pick up an injury and with Pienaar at full-back, we won't lose a goal-kicking option."