But the Bath captain has been backed to come up trumps during England's RBS 6 Nations Championship opener against Italy.
Johnson's retirement from international rugby has left a huge void in terms of his leadership qualities and in the second row.
But Grewcock has produced outstanding recent club form and England coach Andy Robinson has no doubt about his pedigree.
"Danny was selected for the World Cup squad, and we thought that he had a great opportunity going out to Australia," Robinson said.
"But he suffered an injury in the warm-up before the game against Georgia, which was obviously very disappointing for him.
"But he has shown in the past month that he has got back to full fitness, and his performances for Bath have been outstanding.
"Danny has never let England down, and on the occasions when he has come into the team for Martin in the past, he has produced some outstanding performances.
"He played in the Italy game at Twickenham last year, when he showed what a good second-row forward he is. He will be excited about coming into the team, and joining the strong pack that we already have."