Johnson told BBC Radio Cambridgeshire: "I think it would be difficult to take it off him. I've been watching to see how he reacts in adversity.
"He's one of our more experienced players. I trust that he's talking the same language as me out on the field."
The captaincy spent most of last season in the possession of George Boyd, until his ill-fated loan move to Nottingham Forest, at which point goalkeeper Joe Lewis picked up the role.
Gabby Zakuani was forced to sit out Saturday's 3-1 friendly win over Barnet, as he tries to shrug off a groin injury.
The defender, 24, has been told he should be cleared to play within 10 days but the Posh boss has not ruled out bringing in another centre-half.
"He's fit, he's been working hard but he can only run in straight lines at the moment," said Johnson.
"Which would look a bit stupid if he's doing that in games.
"I've got to wait for him, unless I can find someone to enhance the squad."
Striker Dave Hibbert was also missing as he attended the birth of his first child.
But Posh fans can expect to see a similar line-up on the opening day of the season to the one that stepped out at Underhill.
"It's 95 to 100%. There'll be one or two places up for grabs," Johnson added.
"I'll have a look at Bristol Rovers and then I'll pick the team I think will win."
Source: BBC Sport
Source: BBC Sport