Dallaglio, the only man to play in all England's matches when they lifted the Webb Ellis trophy in 2003, is in head coach Brian Ashton's current 40-man squad.
But the 34-year-old faces stiff competition if he is to realise his dream of defending the trophy in France in September.
Johnson, who succeeded Dallaglio when he was axed as England captain, said: "Lawrence has been a fantastic player and a captain of England. If his form dictated that he was the best player then I would play him. If it didn't then I wouldn't.
"Lawrence has done what Lawrence has done, but you are always selected on what you are going to do. I'm not there watching them train. Maybe Joe Worsley, Lawrence Moody and Tom Rees are playing fantastically well and they are the men.
"You have to be good enough to be in the team. But the bottom line is you need to pick guys who are playing the best.
Lawrence hasn't played a lot of Test rugby in the last couple of years. At times he has not been great with Wasps and he has admitted that himself. At other times he has come through.
"But does having him in the team raise the level of everyone else's play and is that worth it? It is an art not a science. That is the coach's job."