Duffy only joined up with his team-mates on Friday, but was thrown straight into the action against Bolton and made his Huish Park bow four days later, keeping Town regular Byron Webster out of the side.
And Johnson admits the 21-year-old defender will only improve with more minutes under his belt.
"He did very well up at Bolton and he did well against Leicester because he's a solid player and a good defender," Johnson said.
"He hasn't had many games so it might take him two to three to get a bit sharper, but he's one we want winning headers in both penalty areas and he can."
Yeovil's 2-1 defeat to Leicester means they are still without a home league point this season, but they were applauded off by the home support, who appreciated the effort and quality their side showed against promotion-hopefuls.
"The fans have been fantastic," Johnson added. "They clap when they see good football and they appreciate the standard of the opposition, which is important to me.
"This is our Premier League and if we survive it unscathed then the whole club will have learned a lot."