Souttar earned a dream move to the then-Premier League club after excelling at last year's World Cup, and played 12 games.
But since the defender's January move to England's top flight, Leicester manager Brendan Rodgers was sacked, the Foxes were relegated to the Championship and Souttar has fallen out of favour.
The 24-year-old hasn't played a minute in Leicester's opening five games of the new season, and was linked to a loan move that didn't materialise.
The towering defender will start in Sunday's game (AEST) against Mexico at the AT&T Stadium in Dallas.
"My message to him is simple and that is, 'This is your stage. This is your opportunity to show your coach at Leicester and ask the question why you're not playing and show the fans how good you really are'," Arnold said.
"He's got to use that as motivation to go out there tomorrow and show everyone that's watching this game who Harry Souttar is. Everyone will remember him from the World Cup.
"There's absolutely zero doubt in Harry's mind where he's at and he's got that great mentality that he wants to show the world what he can do."
Souttar will have a new central defensive partner in Ipswich Town defender Cam Burgess, making his debut for the Socceroos, with Kye Rowles shifting to left-back.
"I was very impressed with him when I went across to the UK (and watched) Ipswich against QPR," Arnold said of Burgess.
"Fantastic guy and he's making his debut at the age of 27 so he's mature and he has had these type of experiences ... at club level.
"With Harry Souttar next to him we've probably got the two tallest centre-backs in Asia by far, because Cam's (194cm) and both of them are very mobile. So it's an exciting time."
Teenage Parma defender Alessandro Circati has "fit in perfectly" and is in the mix to debut as a substitute.
"It's for him to get to learn what national team football is," Arnold said.
"He hasn't played really any junior national team football, he's come straight in with us - but also the environment that we create (is important) and what the expectations are for us.
"But he's a very, very mature 19-year-old. I've been super-impressed with him and there's a good chance he will get some minutes tomorrow."
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