The fitness cloud hovering over United States star Christian Pulisic is growing two days out from a blockbuster clash with the Socceroos.

Pulisic, often dubbed ‘Captain America’ picked up a calf knock in training last week and was withdrawn at halftime of the US’s 4-1 opening win over Paraguay.

The 27-year-old has trained solo all week on a managed plan as the US continue their preparations for Australia at Seattle Stadium on Friday (5am Saturday AEST).

That casts serious doubt over Pulisic’s involvement as a starter, if at all.

“He has not been training fully with the group yet, but we’ve got a couple of days to see where he’s at,” defender Antonee Robinson said.

“Thankfully, we’ve got a lot of boys on the bench who are eager and ready to get involved in helping the team and a lot of quality.

“It’s a long tournament. If we don’t have him back for the game, we’re going to make sure we have him back for the rest of the way because we’re going to need him.”

Attacking midfielder Brenden Aaronson is among those who could replace Pulisic if the star isn’t fit.

“We’re really hoping Christian is going to be back for the game of course,” Aaronson said.

“I don’t really know what is going on in particular, but we know that he is going to give us his all to get back into the team and be there for the game.”

The Socceroos are wary of what Pulisic can offer if fit.

Midfielder Aiden O’Neill indicated it would be down to himself, as a right-sided midfielder, wingback Jacob Italiano and central defender Alessandro Circati - who is friends with Pulisic - to contain the dynamo.

Defender Kai Trewin, who could also be called upon on the right, would love to test himself against the US star.

“I hope he plays,” Trewin said.

“You always want to play against their best 11, so I’m sure he’ll be fine.”

With the AP.