Moeen, however, did bowl in the nets on the eve of Saturday's match and was later included in a 12-man squad announced by the tourists with uncapped seamer Craig Overton added to the starting XI from the opening test in Brisbane.
Moeen, whose bowling was affected by the injury during England's 10-wicket defeat at the Gabba, would probably still be retained as a batsman at Adelaide Oval even if he was unable to bowl, said Root.
"We'll have another look at things after practice and see how he is then and see if there's any more damage to it, and then we'll have to make a decision from there," Root told reporters at the venue.
"His batting's been a huge part of our team for a long period of time now so I think he would still play as a batter.
"He's a top class batter and he's someone who has the potential to go on and become one of the best batters in the world."
Root added that it was too early to confirm a final starting side for the match but one certain inclusion was former captain Alastair Cook, despite a meagre return of nine runs from two innings for the left-handed opener at the Gabba.
"Cookie is a world-class act," said Root. "He's scored over 11,000 test runs, he's been one of our most consistent performers for over 10 years. He's not had the ideal start to the series, but... I'm not going to lose any sleep over him.
"I'm pretty sure that during this series he's going to make massive contributions and he's going to perform really well for us."
With England 1-0 down in the series, Root took some comfort from the damp "English" conditions that he thought might favour the tourists in the first day-night Ashes test.
"(But) I think it's really important that we don't get too excited about that," he cautioned.
"We just need to make sure we perform like we did in the first three days of the first test match for the whole game.
"If we do that and recognise the key moments of the game then we give ourselves the best chance of winning."
England 12-man squad:
Joe Root (captain), Moeen Ali, James Anderson, Jonny Bairstow, Jake Ball, Stuart Broad, Alastair Cook, Dawid Malan, Craig Overton, Mark Stoneman, James Vince, Chris Woakes.
(Additional reporting by Ian Ransom in Melbourne; Editing by John O'Brien)
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