The Blades were only denied a place in the Championship by a penalty shoot-out in last season's final against Huddersfield but they have struggled for form in recent weeks, taking just seven points from a possible 24 ahead of the visit of the Glovers.
And Johnson, who led his side to their highest ever finish this season, insists the pressure is off ahead of the first-leg of their League One play-off semi final..
"We aren't the ones that are expected to win it either at Sheffield United or if we get to the final," he said.
"We are certainly going to be the underdogs, in that way we aren't the ones under pressure because no one expected us to be where we are.
"I think they will be two cracking games but the pressure is on them. They are desperate to get promoted and we know that."
Despite the match being screened live on Sky Sports and taking place on a Friday night the Glovers faithful are expected to travel in their numbers and Johnson admits they could make a difference.
"People know we need them up at Sheffield," he said. "Sometimes if you can get a couple of thousands of your supporters, even if they are in a group with 30,000 home supporters, they can make a noise because they are up for it.
"It'll be nice to take a great band of followers up to Sheffield. I know they'll give us great support and we'll appreciate it."