Walker is on-loan with the Glovers until the end of the season.
"He has come good today and looked that Premier goalie he is going to be," said Johnson. "I find it hard when people moan at keepers.
"If I was to give you a pair of gloves, put you out there and have the lads shoot at you, you would definitely shift yourself.
"When you have got those gloves on you have got to be brave and he was brave today and showed exactly what he had in his repertoire."