The seven-year-old is a Grade Two winner over three miles, but he fell at Musselburgh last time out.
And as the National Hunt Chase is for amateur riders, Greatrex is on the look-out for a suitable pilot.
He said: "He has got a few options but I would say the four-miler is the favourite for him. I think he is a dour stayer. I think the RSA is the harder race.
"He has got a bit of class and I finished second in the race last year with Missed Approach and this horse would be better than him.
"Musselburgh was not ideal but it was only to have a little run round. It looked like he was going very well and Adrian (Heskin) was adamant he would have gone very close.
"He has schooled well since. I'm not sure who will ride but we will try and get one of the big boys."
Missed Approach will be back for another go, this time in the Fulke Walwyn Kim Muir Challenge Cup for amateur riders.
The trainer said: "He has been frustrating this season. He was second in the four-miler last year and, for one reason or another, he has been flattering and not quite got the job done.
"He has had his wind done since Musselburgh. I think he is really well handicapped if things go right for him.
"He obviously likes it around here and Noel McParlan, I think, will ride him and he rode him in the four-miler last year. If we get it all together, he has definitely got a big one in him."
Western Ryder ran well in the Champion Bumper last year and will line up in the Ballymore Novices' Hurdle.
Greatrex said: "Unless things change, he will go to the Ballymore. Richard Johnson is adamant he is a proper stayer.
"He will be a better horse on better ground. It looks a very hot race but he is a very good horse so we will see."
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