The barnstorming Magpies skipper has been battling dizziness during the three pre-season trials the black and whites have played so far in 2019.

Tests revealed the Group 10 representative is battling a form of cardiomyopathy, which basically means some of Rainbow's heart muscles are abnormally thick and that is restricting blood flow around his body when he physically exerts himself.

Rainbow has long been one of the Group's best players but enjoyed a bumper campaign with the Magpies last season, leading the club to the minor premiership and was then later rewarded with the competition's player of the year award.

A member of the 2012 Blues in Waiting squad, alongside the likes of current State of Origin players Jake Trbojevic, David Klemmer and James Roberts, Rainbow is just 26 and entering the prime of his playing career.

He described the news as "a big kick in the guts".

"That's it, done and dusted for good now," Rainbow said, Cowra finishing the 2018 season as runners-up.

"I've got a heart condition and it's gotten worse the last three games we've played.

"I didn't think anything of it at the start but went and got it checked out and the news wasn't good. 

"There's not much I can do about it other than keep an eye on it but there's no football from now on."

The loss of Rainbow a week out from the club's massive trip to Mudgee to tackle Jack Littlejohn's new-look Dragons is a huge blow for the swoopers.

A shoulder injury to Jeremy Gordon has ruled him out for the first seven weeks of the season, too, meaning the club will be without two players that led the Group 10 player of the year race for virtually the entire 2018 season.

Rainbow, though, is very confident Cowra will figured in premiership calculations this season.

He just might not see much of it.

"I'll try to watch depending on how hard it is. I hate watching, I'm bloody shocking," he said.

"I went to the Bathurst knockout and had to go up the tunnel and fill up the water bottles and that sort of thing."

The former Cronulla Sharks under 20s player said his time playing in Sydney was a highlight while helping Cowra to last year's grand final was another.

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Rainbow said the club has plenty of natural leaders to fill the void he leaves, with big prop Blake Tidswell and star back Jeremy Gordon the obvious choices to skipper the black and whites in 2019.

"Big Moz (Tidswell) up front has been good for us for a long time," the former Blayney Bear added.

Cowra will travel to Mudgee on Sunday, while Lithgow hosts Hawks in the first game of the weekend on Saturday.

Also on Sunday, St Pat's hosts Orange CYMS to complete the 2019 opening round after premiers Bathurst Panthers accounted for Blayney 44-10 in an earlier clash ahead of the NRL game at Carrington Park. Oberon has the bye.