Johnson's men ran out 67-3 winners over their opponents in Dunedin, but the coach is already thinking about securing qualification.
Should Scotland defeat Argentina tomorrow, the Scots and England will progress through the group, with the leader being decided when the pair meet next weekend.
A win for the Argentinians would mean that all three teams could still qualify from their final matches.
"It was job done and now we go into a massive week for us," Johnson said after the Romania victory.
"The World Cup really starts in a way. [Scotland against Argentina] will be a scrap, hopefully it will be an incredibly physical game!
"It's a mental thing this week, getting ourselves ready for the game. It's going to be a big one."
England's match with Scotland will take place in Auckland.