Cahill put pen to paper on a short-term deal until the end of the season on Monday, ending weeks of speculation about his future after leaving Melbourne City in December.
The 37-year-old is aiming to become only the fourth man behind Pele, Miroslav Klose and Uwe Seeler to score at four successive World Cups, but for him to accomplish the feat in Russia later this year he desperately needs game time to be assured of a spot in Bert van Marwijk squad.
He watched his new teammates, including fellow Socceroo James Meredith, from the stands at The Den on Tuesday as they played out a 0-0 draw with second-placed Derby to move to 14th on the Championship table.
The fast-paced and furious affair bore little resemblance to anything Cahill would have experienced since leaving Everton in 2012 and Harris hinted he may need to play in a reserve match before being in first team contention.
“He’s not going to be fit for a couple of weeks,” said Harris, who played alongside Cahill in the 2004 FA Cup final.
“He’s not done a lot in the last seven weeks since leaving Oz, so we’re going to have to be patient with him.
“But bringing him in has given us a big lift. It’s been a whirlwind last few days.
“He brings great qualities to the dressing room as a man. He’s played all around the world and he’s great with the young lads.
“He wanted to come back and play for Millwall, he wanted to come back and play for me and he wants to lead.
“It’s stirred the boys up, they are buzzing downstairs they are going to thrive off having him around.”
On a bitterly cold night in south east London, several youngsters in the 11,000-strong crowd sported shirts bearing Cahill’s name.
Australia’s record goalscorer was given a huge ovation from the home fans in his first appearance at the Den since returning for an FA Cup tie with Everton in 2006.
A video showcasing some of the greatest moments from his 251 appearances were shown on a big screen ahead of his arrival on the pitch.
After walking out onto the field, Cahill was hugged warmly in the centre circle by fellow club greats Terry Hurlock and Les Briley with a chorus of ‘one Timmy Cahill’ echoing around the ground.
Cahill posted on Twitter: “Massive thank you to everyone at this amazing football club. Most importantly the fans for making my welcome home to the Den so special and something I’ll never forget.”
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