Manson will train separately with coach Noel Donaldson ahead of the February 13-17 national championships at Lake Karapiro. Trials for the international season will likely pit Drysdale and Manson against each other immediately afterwards.
Drysdale won his first two world championships under Tonks in 2005 and 2006, but was coached by Ferguson to win his 2007 title and the bronze at the 2008 Olympics when he was hampered by illness. Drysdale has since been mentored by Tonks.
Calvin Ferguson told Radio Sport he'll work with Drydale through the summer and re-assess after that.
"He has a challenge ahead for the single spot to get his fitness back up and to see how the speed of his boat goes.
"He's not as conditioned as he can be after a year off.
"I understand what his needs are, and what he's done with Dick. The relationship was quite easy between us all, even though they did their own thing."
Ferguson rowed three years in a national lightweight four with Tonks as coach, and worked as Rob Waddell's training partner at times when the Sydney Olympic champion was mentored by Rowing New Zealand's former head coach.
"I learned a lot of his processes and techniques," Ferguson said.
"Now our children go to sports together and our families get on well. I keep [sporting] politics and individual friendships separate.
"I can have a chat to him - we get on well - and he respects I'm working for Rowing New Zealand, but we haven't discussed it [the Drysdale situation] yet."
Ferguson said Rowing New Zealand needs to have a conversation with Drysdale about who he wants to be coached by long term, provided he can beat Manson.
"That's going to be a good competition. I'm not trying to think too far ahead.
"I'd be happy to coach Mahe all the way through [to a fifth Olympics at Tokyo] because he's pretty much self-managed.
"He knows what needs to be done in terms of hard graft, but I'd be fine as another set of eyes to bounce ideas off."
