The financial woes and several recent changes to Rugby Australia's board meant moves to oust Castle were gaining momentum, the paper added.
The 28-year-old backrow forward is a junior doctor at the Ulster Hospital's Accident and Emergency Department.
Rennie, however, is set to leave in June to become the coach of Australia.
During his studies at Loughborough Flament went from the fifth team to starting for the firsts in English rugby's third tier National One league.
Victory ensured the Springboks made it three wins from three World Cup final appearances.
Despite being the youngest finalists in the professional era, England start as favourites after they dethroned reigning champions New Zealand.
The All Blacks beat Wales 40-17 in the third-place playoff.
South Africa are probably one of the few sides capable of repelling England in their current form.
George is a friend of Jason Roy whose inch-perfect throw allowed wicket-keeper Jos Buttler to complete the decisive run out.
England's first World Cup win over the two-time defending champions puts them into their fourth final -- just four years after their embarrassing first-round exit on home soil.
The defeat brought an end to Japan's marvellous run, in which they stunned Ireland and Scotland en route to topping Pool A unbeaten and reaching the quarter-finals for the first time.
A moment of madness cost France dearly in Oita after lock Sebastien Vahaamahina was sent off for elbowing Aaron Wainwright nine minutes into the second half.
The All Blacks, seeking a third successive World Cup title, scored three first-half tries to all but end the game as a contest, going on to add four more in the second period.
England have now beaten Australia at the quarterfinal stage on all three occasions the Wallabies have failed to reach the last four, after 1995 and 2007.
Fiji haven't beaten Australia for 65 years - a run of 17 winless games against the two-time world champions.