O'Sullivan believes Easterby was rushed back far too quickly and as a consequence did not show his best form until the World Cup warm-up matches last August.
Since then, the Llanelli player has been so consistent that he has become a virtual automatic member of O'Sullivan's squad in an area of the field where Ireland have half a dozen world-class performers.
"Mentally and physically Simon is as hard a player as I have ever met," said O'Sullivan.
"He brings an honesty and endeavour to the back row which I believe is crucial to us.
"His work rate is ferocious. He is incredibly tough and his pain threshold is colossal.
"It took him a while to come back from his injury and I think he was pretty harshly treated.
When he was selected again he wasn't fit enough to do himself justice and his performances were criticised.
"But we saw what he was really about during the World Cup. Steve Hansen told me recently that he reckons Simon is one of the best back-rows in the world at the moment and he would cut off his right arm to get him into the Welsh squad."
High praise indeed for Easterby, who will be winning his 29th cap on Sunday when he lines up for his adopted nation against Wales at Lansdowne Road.
Having spent the last five years in the Principality, Easterby will be a useful source of information for O'Sullivan as he plots the downfall of a resurgent Welsh side who maintained their World Cup form with a clinical defeat of Scotland in their opening game last weekend.
Ireland are looking to bounce back from their 35-17 loss to France in Paris but the home side have been boosted by the return of captain Brian O'Driscoll, who has recovered from a hamstring injury.
"We weren't as far away as many people thought last weekend," said O'Sullivan.
"France didn't play particularly well but we contributed to our own downfall by shooting ourselves in the foot at vital times, which was a bit annoying but it was certainly no catastrophe.
"This game will be another battle because we need to accept that Wales have turned the corner.
"For our part, we have targeted our home games and we need to win to get a foothold in the championship by winning.
"Our next game is England away and it would be very difficult going to Twickenham on the back of successive defeats."