He's not wrong, and this is why. Rennie has done an extraordinary job in turning around the Chiefs' on-field performances since taking over in 2012. They won their first title that year, and another the year after, and under Rennie they have been a tough, stubborn and difficult side to break down; a bit like their coach.
Since 2012, the Chiefs have viewed the Crusaders as a talented team but one nevertheless that possesses a soft centre. Stand up to the playground bully, the theory has gone, and the bully will back down, and really, that is probably the best way to approach an All Black-stacked franchise which has won seven titles.
The Chiefs' success against the Crusaders over the years - they beat the red and blacks in two semifinals in Hamilton in 2012 and 2013 and have enjoyed a good run since until their defeat to their rivals in Suva this season - has been built on a relentlessly physical approach, one that often stretches the laws of the game.
Rennie's side are often on the wrong side of the penalty count and in those early years were often criticised for their tactics of holding and tackling opposition players beyond the breakdown.
As for the Crusaders, under previous coach Todd Blackadder they became annoyed at what they perceived to be niggly and cynical tactics from the Chiefs, and in Crusaders headquarters there was disquiet at Rennie's recruitment practices.
The theory was that the Chiefs took a scattergun approach to signing young players from the Crusaders catchment area - a double-pronged attack that strengthened the stocks in Waikato while depleting those in Christchurch.
The Crusaders built their success in their early years around taking unwanted players from other franchises and turning them into world class players - think Norm Berryman, Norm Maxwell, Ron Cribb, the list goes on, but the luring of promising players either from the Crusaders academy or straight out of school has occasionally angered the southerners.
With so many All Blacks clashing in the sudden death match at AMI Stadium on Saturday, including locks Sam Whitelock and Brodie Retallick, a new chapter in this rivalry is about to be written.
There will be anger and there might even be hatred; required viewing, in other words, for those who like their rugby to be relentless, skilful and, above all, intense.