"He has got everything to be a top, top player - pace, great in the air, can pass with it and run with it," Redknapp said. "He's getting better and better and his confidence is good.
"His concentration has improved and most of all, his confidence has."
Redknapp feels Johnson is benefiting from regular games, compared to his situation at Stamford Bridge where he had to seize every opportunity to impress.
"If you made a mistake you were out the next week but here with me, unless he has a disaster he is playing every week because he's different class," the Pompey boss said.
"They had so many players. You might think you'll get dropped next week again if you make a mistake, and (Paulo) Ferreira would come in."