"Every test match is a trial for everyone," he said.
"We got a massive challenge last week from the Irish and we have to stand up to that challenge this week and Liam is only one of the 22 people who have to do that."
The selectors considered switching him with Richie McCaw but felt Messam's best position was six and he needed to concentrate on that role while McCaw had worked with the team calls for the last few tests and could adapt.
Messam has played just nine tests and missed the last World Cup squad but never thought about shifting overseas.
"One of my traits is I take it on the chin and move forward. You can't live in disappointment otherwise you won't have a happy life so I just take it on the chin and move forward with the next thing in my life."
Injuries to Read and Daniel Carter created more disruption than Hansen and the selectors anticipated.
But they have picked Luke Romano to debut at lock, Sam Cane for a start on the openside and Messam on the blind in their pack changes. Behind the scrum, Aaron Cruden will direct the side from first five-eighths while Hosea Gear and Ben Smith are the new wings.
Read's return to the game is uncertain because of concussion while Carter will miss several weeks because of a hamstring strain.
"Right from the word go we felt if this series had been wrapped up by Christchurch then we would look to make some strategic changes," Hansen said.
He also explained his rationale with Ma'a Nonu.
He had not had enough rest after 18 months non-stop rugby so the selectors decided to repair him now so he was sharp later in the year.
"What's happened in Christchurch is that Ireland have given us a good tap on the chin and said 'heh, you better front up or we'll take that [unbeaten] record off you'," Hansen said.
"So the challenge has been laid out there for us and I'm sure Ireland will be coming here with a huge amount of confidence and desire to try and win the game.
"We've got to meet that and play as well as we possibly can and that's about getting things right from Monday to Friday and getting them dead right, not pretend right."