In a stern message to his players, the Englishman has hinted at possible changes to the starting line-up in the crunch match.
“I have a tough choice tomorrow (Wednesday) but that's all down to how hard the players are wanting to wear the green jersey of Gor Mahia," Kerr said.
The two sides are separated only by six points, though Gor Mahia are top of the league on 40 points, but Kerr has challenged his players to show character in the clash.
"The game against Posta Rangers will be a tough game for us because of the high expectations of everyone. But look at what the league position shows now to what it could show at 6pm tomorrow."
Posta Rangers will be looking to do better than their 1-1 draw against K’Ogalo when the two sides came face to face in the first leg at Kasarani.
A win for Gor Mahia will surely end Posta Rangers hopes of clinching their maiden Kenyan Premier League title this season.