Van't Schip said he looked abroad after deciding he couldn't find a gloveman of Sorensen's quality at home.
"Looking in Australia, we didn't think there was one of the calibre of Thomas available," he said.
"Tando's been doing a good job but the levels have to go higher for everybody."
Sorensen linked up with the club for the first time on Tuesday, watching his teammates train at the City Football Academy after flying in from Europe.
In his first interview in Australia, Sorensen said he said his first look at training and the team's City Football Academy base convinced him the team could be successful this season.
"The more I heard about (City), the set-up, the ambitions, it was everything I was looking for, a great challenge," he told AAP.
"(There are) no excuses for myself and the team."
Sorensen's age - he turns 40 next year - has raised eyebrows but the Dane has the impressive feat of cycling across America just weeks ago to show his fitness.
"I'm very serious about my sport and when you get to my age you've got to look after yourself," he said.
"So I'm in good shape but I need to get back into football mode."
He said retirement was a "consideration", after leaving Stoke City, but he dismissed reports he announced a departure from the game.
"Somebody did it for me, papers are quick to jump to conclusions," he laughed.
"I've had a fantastic career in England, some fantastic times and great clubs ... but sometimes you need new challenges in your life to keep going."
Sorensen's arrival ends well-liked Argentinian utility Jonatan Germano's time at City, with van 't Schip saying he would try to find him a new club.
"We had to make this decision that's not nice, but it's the world of what we live in," he said.
-AAP
