Moyo, 20, was handed a start as one of four debutants against the Welsh in Durban, where the Springboks scored seven tries in difficult conditions to make it three bonus-point wins from three games this month.
"He definitely played well," said Erasmus, adding that the player's nervousness and the wet conditions contributed to some of his kicking lacking its normal accuracy.
"I thought he handled it extremely well. But I’ve been there; in your first or second test, you’re tired because of the nerves and I thought his tyres were a little pap (flat) in the second half when we took him off."
Feinberg-Mngomezulu’s return from injury is a boost to the side, with Erasmus also confirming that experienced flyhalf Handre Pollard is struggling with a hamstring issue that will keep him sidelined.
The Springboks were sloppy at times against Wales and wasted numerous scoring opportunities.
"It was tough to play an expansive, pretty game in weather like that against a team that really wanted to defend," he said.
"But we didn’t lose a lineout ball and, while we weren’t on our game in the scrums last week, we were good today."
Erasmus also highlighted South Africa's mauling and maul defence.
"We had a lot of fixes that we got right," he said.
"Our defence was better than last week (against Scotland), we worked really hard on that, and then our set piece was really solid. I think a lot of guys came through this test with ticks against their name."
The Springboks face Argentina in a one-off test in Buenos Aires on August 8.
"Argentina is going to be tough, they’ve had our number a few times. We are just trying to build our game and I think today was another step forward," Erasmus said.