Brisbane Roar head coach Ben Cahn has taken indefinite sick leave on the day the A-League Men club's star signing Marco Rojas was unveiled to the media.

In a sad turn of events ahead of the Roar's home clash with Wellington Phoenix on Friday night, the club announced three hours after Rojas was introduced to the press Cahn was taking indefinite leave, effective immediately.

"It's an incredibly difficult decision to step away from something I've worked so hard for, but my health and my family come first," Cahn said in a statement.

"I'm very grateful to management for supporting me in this decision and have no doubts that they'll continue to take the club forward."

Newly appointed assistant coach Ruben Zadkovich will take over as acting head coach against the Phoenix.

Zadkovich will be the Roar's fourth head coach this season after the earlier exits of Ross Aloisi and Luciano Trani, who lasted just one game before being axed.

Star forward Rojas joined the Roar this week from Chilean club Colo-Colo.

The player known adoringly by Australasian fans as the "Kiwi Messi" played 140 games for Melbourne Victory and Wellington, scoring 41 ALM goals. He has also represented New Zealand 43 times.

Rojas said he had been training with the Roar for several weeks and he was ready to assist the club re-enter the top six after slipping from near the top of the table two months ago to ninth.

"It is not the only reason I have come here but we would all love to have success," Rojas said.

"I think success for us this season means being able to make the top six considering where we are. If I can help contribute to that it would be awesome."

Rojas explained why playing a full game against his table-topping former club would be "out of the question," but he is expected to come off the bench.

"My last game was mid-June and that was 45 minutes so I'd be very surprised if I was able to run through 90 minutes," he said.

" The main thing is that I can have impact wherever that is.

"I took a sabbatical for half a year because it is what I needed to do personally and for family reasons, so I am just looking forward to being on the pitch.

"What a special occasion to start again. It would be nice to get a run on against one of my old clubs. It is also awesome to see them doing so well."

Rojas said he was unsure what position he would play.

"I have played various positions. As long as it is in the attacking third I am pretty happy," he said.

"I have always liked playing No.10 but I have had special moments on the wing, up front and in Europe I played No.8 at one stage."