The former Adelaide United and Western Sydney forward arrived at Pompey last season, helping the club gain promotion from League One by registering 13 goals in 31 games.
But following Portsmouth's move to the Championship, Yengi has failed to score across just 14 games after battling hamstring and knee injuries.
Socceroos coach Tony Popovic is a noted admirer of Yengi and has identified the 26-year-old as a player who can be a point of difference for the national team.
But Popovic will find it harder to pick the powerful striker if he doesn't secure a move where he can play every week.
"He does offer something different to any other striker, any potential striker that we have," Popovic said earlier this month.
"There's no one that has that frame or body. I'd love him to use it more.
"I'd love him to impose himself a lot more and actually impose himself on the defenders.
"But I think he needs to find somewhere to play regular football to do that … That's what I would like all the strikers to do: find an environment where you can play regularly, where you're an important player.
"And if you're scoring goals, then regardless of where it is, it's something that we're always searching for."
Pompey had an extension option in Yengi's contract but chose not to trigger the clause, allowing the striker to secure a move elsewhere.
Fellow Australians Jacob Farrell, Tom Waddingham and Hayden Matthews remain contracted to Portsmouth, who announced Yengi was one of five players set to leave Fratton Park over the off-season.
"We would like to thank all those players leaving the club for their service and wish them nothing but the best for the next stage of their careers," said manager John Mousinho, whose side finished the season in 16th spot.
"Kusini played a massive part in our memorable League One title success just over a year ago."
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