Cahill rejoined the Lions last month and the scored a goal for the club's Under-23s a week later.

Last Friday, the striker came off the bench for the first-team and played the final four minutes in a draw with Cardiff City.

Before his return to Millwall, the Australian's last game was against Newcastle Jets for Melbourne City back on December 2.

Lions boss Harris admitted that Cahill's role is likely to be as an impact player unless he can improve his fitness quickly.

"He needs a lot more minutes on the training pitch and in behind closed doors games to really get him up to speed," he told BBC Radio.

"If he gets up to speed in the next few weeks and is ready to start a game then fantastic. If he is not, then he has got a massive part to play from the bench."

Millwall are currently 14th on the Championship ladder and face Birmingham City this Saturday.