Adelaide United will officially unveil key defensive signing Mark Rudan on Thursday as the hard man so urgently needed ever since the loss of Sasa Ognenovski to Asia.

Rudan captained Sydney FC to the inaugural A-League championship in 2006 before going on to play in Japan and Lichtenstein.

The 33-year-old will resume a defensive partnership with fellow former Sydney stopper Iain Fyfe in the heart of the Adelaide defence.

Both will compete for a place with Robbie Cornthwaite and Daniel Mullen, as coach Aurelio Vidmar seeks the right balance to replicate the outstanding defensive success of 2008-09.

Ognenovski's strengths were as a big body, an enforcer and an organiser at the back, while his aerial ability in concert with Cornthwaite made Adelaide a considerable threat from any set piece.

"It hasn't been easy, because we need strength all over the park but that's one area where when we had Sasa there it (strength) wasn't a problem," Vidmar said on Wednesday.

"But at the last minute he decided to leave and that really threw the cat amongst the pigeons.

"A lot of Australian boys in that position were already taken and we had gone for three of the best in the A-League in those positions and for whatever reason they declined.

"We need a balance, last year Sasa and Cornthwaite worked well because Sasa was aggressive and Corny was a little bit different, so we can't have two of the same players in that area.

"Fyfey brings us something different and Corny hasn't been able to do anything in the last couple of weeks due to his illness and will need time."

Rudan joins a defence that conceded two goals from corners in their most recent friendly outing against Melbourne, underlining the work that still needs to be done ahead of Friday nights' season-opener against Perth Glory.

"We need to work, it was a good thing because we changed our set-up in the pre-season games at set pieces to have a look at a few different things," said Vidmar.

"Certainly it's great that it's actually happened in pre-season and we've got some time now to fix it up before the first game."

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