"Davey hasn't had a great series, there are no secrets about that," Langer was quoted as saying by cricket.com.au.
"But he's also a word-class player and I've said throughout the whole series, if Davey has one good innings it'll help us win the Ashes. He probably hasn't been through this lean run before, so it's going to be a good test of his character.
"But he's a match-winner and he's been brilliant around the group since he's been back (from suspension)," he added.
Terming Warner a "match-winner", the Australian head coach said that he is looking forward to see the 32-year-old bat at The Oval.
"He's been great, good for the crowds, good amongst the group.
"He hasn't got the runs he wants at the moment but jeez I'm looking forward to seeing it when he does, and there's no better place in the world to bat than The Oval."
Australia have an insurmountable 2-1 lead in the series, retaining the Ashes, and will look to win the rubber in England for the first time since 2001.