Viadana take a slender two-point advantage into the second leg of their quarter-final but Rotherham were boosted by their comeback in Italy last weekend that produced 24 points to stun Viadana.
"Viadana were a lot of people's favourites for the Parker Pen Shield so, to be realistic, to come away from the away leg only two points adrift should have been most pleasing," said Johnson
"But, being greedy, I was not that happy. We had opportunities to win the game and the fact that we did not was disappointing.
"However, I am confident we can make up the necessary points at Millmoor and go through to the semi-finals in April.
"It would mean the world to me personally and the club in general if we can win through to the last four - and possibly the final.
"We have struggled in the Zurich Premiership and if we could have a prolonged cup run in Europe that would be huge and provide a great lift for the club.
"I thought Viadana were a pretty well drilled side but their real strength is in their backline. They have got some big lads who hit it hard up the centre and they certainly did some damage last weekend.
"They are a very good team and they had all their leading players in the side but I am sure they will want to improve their ball winning in the return leg.
"As far as we go, we will need to show the same courage and determination as in the game against GRAN at Millmoor - and in the final 30 last weekend when we turned a 32-6 Viadana lead into that final 32-30 scoreline."
Mario Savi, the Viadana captain, rues letting slip that significant advantage in the later stages last Saturday and is calling for the full 80 minutes from his team this time.
"We may have beaten Rotherham but the points difference is much smaller than we would have liked," said the Viadana prop.
"We made gross defensive errors in the second half which cost us dearly. We were down to 14 men due to the sending-off of our scrum half Mazzantini and our concentration levels dropped.
"It's going to be tough at Rotherham, a difficult challenge. We have to improve on our performance if we are going to win on Saturday in order to go through.
"We have to play at 100% for 80 minutes - which we didn't do on Saturday - and not make the same mistakes that crept into our game. We have to be more focussed from start to finish.
"Rotherham have a solid scrum and quality backs. They don't really have a weak area. However, we have to be positive, we can't go over there thinking we are going to be outdone. We are going there with no pressure with a desire to play and do well."