Carr, who turns 29 on Wednesday, was a regular standout in showdowns of the past, but has struggled to recapture that form since returning to Alberton in October.
The ex-Fremantle vice-captain was serviceable in the win over Hawthorn in round four, but picked up just nine possessions against St Kilda last week.
Williams revealed Carr has been carrying a hip complaint, but admitted the gritty onballer was on his last chance to stay in the team.
“Josh’s been a little disappointed with how he’s played, but he does add a lot of knowledge, leadership and toughness to our side and we like that about him,” Williams said on Wednesday.
“Obviously, he needs to continually build on his game and where he is now is not what we’re going to be satisfied with.
“There’s no guarantee he’ll be in the side this week”.
Carr forged a reputation for antagonising Adelaide players during his first stint at Port Adelaide.
The fierce rivalry culminated with an off-field showdown between Carr and former Crows’ skipper Mark Ricciuto outside the Ramsgate Hotel in 2002.
Ricciuto’s successor Simon Goodwin said his side would be ready for Carr’s “antics” in Showdown XXVI on Saturday night.
“Josh has been a fantastic showdown player and we’ll expect no different from him this week,” Goodwin said.
“He’s hard at the footy, creates a presence on the ground and we look forward to renewing that rivalry. He added a lot to the showdowns when he played.”
Carr isn’t the only Port Adelaide player on notice.
Williams, who put his charges through a gruelling session in the rain on Monday afternoon, said the players understood last week’s 66-point loss to the Saints was unacceptable.
“Based on last week I reckon there are 18 players that better be training well today,” he said.
“A few of the SANFL players are really looking for the game and the guys know I’m not satisfied with how we played last week.
“I guarantee you’re not going to see the rubbish you saw last week.”
