England was supposed to field the same squad which is, currently playing the second warm-up match, in the first Test but Fletcher said Trescothick's departure for stress-related ailments had weakened the batting depth and Monty's poor batting might go against him.
''We wanted Monty if Trescothick was there, because Trescothick added (batting) depth. Now, we might have to re-think it,'' Fletcher told BBC Sport ahead of the first Ashes Test which begins in Brisbane on 23 November.
England is likely to go with four pace bowlers and a single spinner in the match and according to Fletcher, Giles' batting ability gives him in edge of Monty.
If he is picked ahead of Monty, this would be Giles' first Test in a year after the left-arm spinner had undergone three hip operations.
Panesar is more an attacking bowler than his senior teammate and the Indian-born left-arm spinner also showed commendable control in the practice match.