Sydney castoff Armstrong Jones is under an injury cloud which threatens to derail his first middle distance test this campaign at Eagle Farm on Saturday.Trainer David Murphy accepted with the five-year-old and stablemate Deacon for the Cullen Handicap (1800m) but has doubts whether Armstrong Jones will pass a fitness test on Friday."He struck himself behind his off-front knee playing in the sand roll yesterday," Murphy said."There's not too much swelling there at the moment but he's a big horse

Sydney castoff Armstrong Jones is under an injury cloud which threatens to derail his first middle distance test this campaign at Eagle Farm on Saturday.

Trainer David Murphy accepted with the five-year-old and stablemate Deacon for the Cullen Handicap (1800m) but has doubts whether Armstrong Jones will pass a fitness test on Friday.

"He struck himself behind his off-front knee playing in the sand roll yesterday," Murphy said.

"There's not too much swelling there at the moment but he's a big horse and I don't want him to hurt himself any more.

"It's a shame because he's on fire at the moment.

"I've accepted with him but he's missed some work so I'll take him to the beach tomorrow and see how he goes before deciding if he'll run or not."

Murphy, a former champion apprentice in Queensland in the mid-1980s, planned to test Armstrong Jones over a middle distance following his impressive victory over 1615 metres at Doomben last month.

"If he doesn't run Saturday I'll give him a break and bring him back in the summer," he said.

"There's two Listed races, the Bernborough Handicap and another race, which I've got my eye on with him."

Armstrong Jones began his career with Gai Waterhouse in Sydney but was bought privately for $16,000 and transferred to Murphy who spent four years working for the master Bart Cummings in Melbourne.

"I went to Melbourne and rode over the hurdles after I left Queensland and I worked for Bart for four years and then I worked for another five years in Dubai," Murphy said.

Murphy began training in Queensland with a small team five years ago but his stable numbers have grown to 16 in work.

Armstrong Jones has only been under Murphy's care for six starts and his recent Doomben victory was his second for the stable.

Christian Reith will retain the ride on the son of Catbird if he's cleared to start while Larry Cassidy replaces the in-form Stathi Katsidis on Deacon.

Katsidis won on Deacon two starts back in a 2200-metre class six at Eagle Farm and rode him last start when the Zabeel six-year-old finished fourth to Malandrina in a 2200-metre Class Six Plate at Doomben last Saturday.

"Deacon is dropping back in distance to 1800 metres but I think that will suit him," Murphy said.

"He needs pace in his races and I think 1800 metres is his best distance."

Although the stablemates are unlikely to clash for the first time at Eagle Farm, a confrontation looms at some stage over the summer.

"I'll probably spell Deacon after Saturday's run and I'll get him ready for the Bernborough as well," Murphy said.

The Listed Bernborough Handicap (1615m) is the final leg of the Doomben summer series in January.