A sickening collision at training has put Adelaide United defender Daniel Mullen in doubt for Friday night's clash with bitter rivals Melbourne Victory.

The right-back, who turned 21 today and signed a new two-year deal yesterday, landed awkwardly during a tackling drill with midfielder Lucas Pantelis and had to be helped to the dressing room.

It is unsure just how serious the injury is but at this stage at looks as though he will miss the trip to Melbourne along with Mathew Leckie (knee) and Travis Dodd (achilles).

The league leaders meet the Victory this Friday, October 29 at Etihad Stadium - kick-off is at 8:00pm.