Callum McFadzean scored the only goal of the game in a 1-0 win for the Blades at Bramall Lane but Yeovil had their moments and, with home advantage to come on Monday, Johnson is optimistic.
"When it's two legs you're not out of it, that's for sure," he told Sky Sports. "I'm disappointed with how the goal came about and it probably wasn't a beautiful game.
"But we gave it a go after the break, showed a bit more bravery on the ball and had our chances.
"We were a disappointed we didn't get at least one of the (penalty) appeals but it's not counted and we go into the second game one goal down which is a fantastic position to be in.
"We played our better stuff after the goal and being one goal down at Huish Park with our fans behind us making that noise I'm sure we can get that back."