Johnson scored his first goal in more than two years on Wednesday night as City thrashed Scunthorpe 5-1 in the Carling Cup.
Hughes said: "If we can keep him on the field of play, then he's like a multi-million pound signing. He's just one of those players who looks like he's got all the time in the world.
"That's because of the ability he has to create space for himself. And the timing and quality of his passing is excellent as well.
"He's one of the outstanding young players in the country in my view."