The former Ipswich Town midfielder headed in five minutes from time to seal a 2-0 victory for Yeovil over Sheffield United, sending them through to the final of the League One playoffs in the process.
However, Johnson has admitted that he had been left disappointed by the 23-year-old's performance during the first leg last week.
"We've been talking about heroes and villains and there was a villain out there who turned into a hero which was fantastic," he said. "[Upson] was one of those who had a bit of a lack of concentration in the first-leg that led to their goal.
"He thought I might have left him out all week, along with a couple of others. But I gave those villains the chance to be heroes and that is what they did."
Upson signed for Yeovil back in 2010 from Ipswich.