Mitchell's future has been a hot topic of conversation in the NRL with the Sydney Roosters confirming on Tuesday they had withdrawn an offer to the Kangaroos centre for a contract extension beyond 2020 worth $800,000 a season.

There has been speculation Mitchell could be released immediately from the final year of his deal because of the impasse but despite a strong relationship with the young centre after working with him last season, O'Brien ruled out the possibility of signing him for next season even if he becomes available because the club does not have the salary cap room.

It's also highly unlikely the club will find the money for 2021 but O'Brien claimed you never say never to signing elite players.

"You can never put a line through elite superstar players but at the same time, it is a long way away from happening just due to the amount of money that you'd have to pay for Mitchell," O'Brien said.

"Any team in the comp would be lying if they said they wouldn't be interested in him but the salary cap - that's the biggest variable in this.

"He's an exceptional talent and I connected really well with Latrell but having said that, there are a lot of things to get through in terms of our own."

O'Brien is fully aware of the emergence of young stars such as Bradman Best and Starford To'a.

"You've got some young guys here that don't necessarily want a road-block either. Their development is really important to me. I think we have some good kids here," he said.

"Getting the right balance in your cap is important. At the moment, we've got a really good spine, we've got some good guys up front in the forwards but you can't have everyone, that's why there is a salary cap. We've got young guys like Bradman Best to consider."

The Knights kicked off pre-season training on Wednesday morning under the watchful eye of their new coach with a short, intense field session.