Goals from Leon Clarke and Carl Baker inflicted defeat and ensured Bury currently sit five points adrift of safety in the League One table.
"All the games left are tough but some are tougher than others," assistant boss Shirtliff said. "We're looking at games we can target and games we think we can pick up points from. Being realistic that's what you have to do.
"We wanted to be a little bit more offensive and, at times, it looked like we were causing them problems. They're a good side and we caused them problems but didn't take our opportunities and, at the end of the day, that's what you've got to do.
"We won't come up against that quality every week. We've got to roll our sleeves up and get stuck in against the teams in the bottom nine and we're more than capable of getting results against them."
Bury lacked cutting edge, a big reason for their signing of former Hull striker Craig Fagan, who made his debut as a late substitute.
"Fagan's signing is just to give more firepower if we can," said Shirtliff. "We need those extra bodies to add to the squad.
"This is it now. I don't think we'll sign any more."