Tierney accepts the review will have to be done relatively quickly with a board meeting planned for 17 December but does not want it to be a "knee-jerk assessment".
"We do them after every series, we are just recognising this one needs to have even more input into it from a usual one because we are in an extraordinary position," said Tierney.
"If there were quick fixes, we would have made the quick fixes, so it has got to be about what are the sustainable changes we can make."
Collier-Keywood also admitted the WRU would have to initially put in a temporary solution if Gatland departed.
"There's no way we'd be able to recruit a head coach of Warren's standing for the Six Nations given the time frame so we'd look to put an interim head coach," said Collier-Keywood.
Tierney added: "That's part of the decision. They're not all stood waiting in a queue.
"A lot of very good coaches are already under contract so that has to be part of it.
"What choices do we have available?"
Collier-Keywood admitted it was important they made the right decision and says the WRU were conscious of the losing run being extended.
"We've got some hard matches to play in the Six Nations," said Collier-Keywood.
"But equally we have some time with the players moving forwards and I'm sure they will have learned a lot through this Autumn Nations Series.
"We've had a few injuries which I don't think helped and we're hopeful they'll come back and Warren will have a larger squad to pick from, if he's there."