"He has been working really hard out there. I've got no worries at all," Johnson said of Clarke's left hamstring.
"He's said he's 100 per cent fit and you go by that.
"There's going to be no fears at all.
"He's always been a hard worker. You've got to put trust in him."
Clarke was cast in a poor light during the first Test of the 2013-14 Ashes when he told James Anderson to "get ready for a broken f***ing arm".
But public opinion has completely turned over the past fortnight, since the death of Hughes.
Clarke fronted the press on behalf of Hughes' family and the national team, then delivered a touching tribute at the funeral.
Clarke, a close friend of Hughes, has earned widespread praise for being so mentally strong in a time of much trauma.
"I've definitely seen a different side to him," Johnson said.
"It has been an emotional roller-coaster and how strong he has been (has been impressive).
"We need him out there in this tough series."
Johnson explained Clarke "just has something about him when he's out there as captain".
"Everyone listens," he said, filling in for grief-stricken Clarke in the pre-match press conference.
"We just want him to score runs and be that aggressive captain."
Shane Watson suggested on Monday there was "no doubt he'll be ready to go in such an emotional Test"
"I'd be extremely surprised if he didn't peel off another hundred because he's certainly got a lot to play for, as we all do," Watson told the Nine Network.
- AAP