A devastated Tony McEvoy has barred stable jockey Matthew Neilson from riding his horses outside South Australia after he weighed in almost 1kg over on Mouro at Moonee Valley on Saturday.

And the ban could have an immediate effect on Neilson who would otherwise have ridden the McEvoy-trained import Le Roi at Caulfield on Saturday.

McEvoy didn't realise until well after last weekend's race that Neilson had returned to scale heavier than when he went out on Mouro who went down by only a long neck to Lord Of Brazil at the Valley.

"I was devastated when I found out," McEvoy said.

"It certainly cost us the race.

"As a result Matthew won't be riding my horses in Melbourne."

Stewards fined Neilson $800 for carrying the extra weight which the jockey claimed resulted from a drink he had after weighing out.

McEvoy described the incident as "unprofessional" and given the success he's had in Melbourne this season, his penalty is bound to be more harsh than the one imposed by the stewards.

Le Roi, like Mouro, is one of an impressive team of imports McEvoy has produced this season.

And like Mouro he has his share of ability.

Le Roi won in France at his only start before joining McEvoy who gave the horse one run in his initial Australian preparation in April this year.

After a short spell Le Roi has returned to win in Adelaide at his only two appearances this time in and will be put to the test in an open handicap over 2000m at Caulfield.

McEvoy made up his mind to give Le Roi the most severe examination of his racing life after the horse worked well at his Kildalton Park training complex on Tuesday.

"I'm very, very pleased with him, he's a real racehorse," McEvoy said.

"We've been learning about him all along and I think this trip to Melbourne will tell me a lot more."

Le Roi's Adelaide wins have been over 1800m and 1950m and McEvoy believes Le Roi will be even more effective in longer races.

"He's only a three-year-old by northern hemisphere time and his first win was in a 1600-metre two-year-old race," he said.

"So there isn't much doubt that he'll really stay."