The 21-year-old former Arsenal trainee was a constant threat down the Glovers' right flank and Johnson confirmed that it was not an injury that triggered the defender's withdrawal.
"We brought him off because he looked like he'd run his race, and had done well," he said. "I thought he played well, he got up there and he was a right-back and a right-winger. So we were pleased with him, and I'll let him know that.
"No pros like coming off, to be honest, but they know when I'm disappointed when I take them off, and they know when I'm not disappointed. So he'll be okay."
The 2-0 victory against the 10-man Chairboys was sealed thanks to two first half goals from Ed Upson and then Sam Foley but Johnson was frustrated with his side's second half display.
"It ceased to become a competitive game until probably the last ten minutes, when Wycombe gave it a bit of a go," he said. "Our lads switched off a little bit, which was a bit disappointing.
"Not all of them, but anyway I'll deal with that, as and when I need to. In time, when I write my book, you'll get to know what I really felt about the last 20 minutes!
"Some of the lads that are in that dressing room deserve to be there, (in the next round) but one or two don't."