Moody, 33, returns to lead England in their Investec international against Wales after missing the RBS 6 Nations title-winning campaign with a knee ligament injury.
After the tournament, Johnson confirmed Moody was the man he wanted to lead England into the World Cup, fitness permitting, but there was a clear change in tone on Friday.
Tom Wood and James Haskell excelled in Moody's absence during the Six Nations, adding to England's stiff back-row competition, while Mike Tindall led the team to four victories.
On the eve of England's first World Cup warm-up international, Johnson warned Moody that nothing is set in stone.
"I've always said guys have got to get their place in the team," Johnson said.
"If any player isn't playing as well as another guy then is that other guy going to play? Well, yes.
"When we name the squad we will name our captain. Everyone is judged on merit and one of Lewis' merits is his leadership.
"You need that quality in the team and I think we missed him and Mike in the final Six Nations game in Dublin.
"Lewis, being who he is and what he has done, brings a huge amount of intangibles to the team but also adds to the collective."
It would be a major surprise if Johnson did not name Moody as his World Cup captain when the final 30-man squad is announced on August 22 after back-to-back games against Wales.
Moody's leadership credentials were endorsed earlier this week by Tindall, who revealed the Bath flanker "is viewed in my eyes and everyone's as the skipper".
But England's back-row competition is arguably stronger now than at any time since the World Cup-winning year in 2003 with Haskell, Wood, Chris Robshaw, Hendre Fourie and Nick Easter all competing for places.
Moody understands the lie of the land. Having played just three and a half games for Bath since being injured the veteran international is itching for a taste of the action again.
"I am excited about this. Everyone is fired up and so they should be," said Moody. "Every game leading into the World Cup is a trial for the squad and the individuals.
"I have always felt I have to prove myself, no matter what stage I am at in my career. There is always competition and that is what makes for a good squad.
"You have to make sure you are playing as well as you can do - and if you are captain that is an honour on top of that."
Moody will captain a changed but strong England team that features Riki Flutey, Delon Armitage and Jonny Wilkinson in the starting line-up for the first time since the 2010 Six Nations.
Matt Stevens has been absent from the squad for nearly three years after being banned for cocaine use while the powerhouse Leicester centre Manu Tuilagi will make his debut.
"Everyone is ready to play. I have had to calm everyone - and myself - down this morning," said Johnson, whose squad have been in camp for six of the last seven weeks.
"We are ready to go and we are excited about it. I want us to go and hit our straps, put in a good performance. Practising winning Test matches is what we are here to do.
"We want to be battle-hardened. The 30 that go to New Zealand need to have two games under their belts.
"Whatever you do in training it is nothing like playing in a game because the adrenaline makes the tempo incredibly high.
"This game is not a send-off, it is a Test match and World Cup positions are at stake. The players need to produce good individual performances and work together as a team.
"In that sense it is the same as if it is the World Cup semi-final. It all comes down to performance."
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