The Millers currently trail the league leaders by five points with West Didsbury having a game in hand.
And the joint-boss knows New Mills cannot let that gap increase if they are to keep their title hopes alive.
“It is a must-win game and there is no hiding that fact,” he said.
“If we lose the game it is not the actual end, but it would give them the momentum to go on and be successful.
“It is a big game. I'm not always one for bigging up games and it’s three points, but we can’t hide away from the fact that we have to win.”
But it is a game in which Birch has full faith in his boys.
“We can beat West Didsbury. We drew 2-2 at their pace and deserved to win,” he added.
“They are a good side, they are not top of the league for nothing, but I'm confident that we can beat them.
“We are at home and I expect us to be favourites to win the game. It is up to the lads, they have been brilliant and I know how much it means for them to be successful.
“They are all looking at the table and they know what a big game it is.”
They go into the big match on the back of a 2-2 draw at Stockport Town in which they had to come from two down.
“We thought we started the game well and then we got hit twice and then we were up against it,” said Birch.
“We showed great character to get a draw and overall it was probably a fair result. It was a tough game for us. There are no easy games.”