Reilly was one of the competition's best backmen in 2012 and was selected in the initial All Australian squad.
The 29-year-old has struggled with form for much of 2013 though, with his average number of disposals a game (18) at a five-year low and his five marks a match are down from six a game last season.
Assistant coach Mark Bickley pulled no punches in his assessment of Reilly's form and the reasons for his omission from the team to face Richmond on Saturday.
"The expectation as a defender is you have to be responsible for a player and defend and we feel he hasn't quite done that to the level that we expect of him," Bickley said.
"Players go through peaks and troughs and if you go through the 40 blokes who were nominated for All Australian last year, I'm sure that they're not the same 40 that are nominated this year.
"Players jump up and others drop off and he's just dropped off [from] where he was.
"If you do a really simple comparison of what he was able to do last year and what he's doing this year in terms of being able to shut his man down and get off and intercept – he probably hasn't been at that [same] level."
Reilly won't play SANFL football for Sturt this weekend, meaning he'll have an extended break to "recharge the batteries" given Adelaide has its bye next weekend.
Bickley was in no doubt Reilly would force his way back into the senior side.
"He'll get a couple of weeks now just to reset, to work on some things we'll train him here and give him some assistance with it," he said.
"He's really keen to obviously get back in the side, he's not in denial or anything like that – the character of the bloke will say he will get back in the side.
"He'll work really hard over that two weeks, recharge the batteries and he'll come back."
Scott Thompson passed his second fitness test in as many weeks on Friday and the club cautiously said he'll play on Saturday.
Thompson battled through a corked thigh last week to play the Sydney Swans, although the injury looked to hamper his output.
Bickley said the 30-year-old star felt tightness in his leg as the match progressed but that as a professional he was used to playing with injury.
If Thompson doesn't pull up well enough from training tomorrow morning, as happened to David Mackay last weekend, Bickley said Brad Crouch and Jarryd Lyons were on hand to replace him.
Harry Thring is a reporter for AFL Media. Follow him on Twitter: @AFL_Harry.
