"My biggest fear in this World Cup was coming up against your old team and how that can impact you and make you go a little bit crazy in decisions," Herdman said.
"I made a promise that I wouldn't let my ego get in my way and this wouldn't be about me beating my old team, just getting six points."
Canada, ranked eighth in the world, is coming off a 1-0 win over No. 16 China while No. 17 New Zealand was beaten 1-0 by the 12th-ranked Netherlands in the other opening Group A match.
The Ferns dominated possession for large spells against the Netherlands but struggled to create many clear goal scoring opportunities.
"We feel we have a real strong understanding on how Canada will play," said Readings.
"Every team here knows about their opposition, it just comes down to who can execute the best on the day - that's going to be the challenge tomorrow."
"The fact we are playing against a coach who used to be in charge of this team doesn't really change too much for us."
The top two from each group progress to the Round of 16 as well as the four best third-placed finishers.