The former Manchester City forward believes that, despite losing his place at club level, the 26-year-old will come good ahead of the World Cup finals in Brazil next summer
Hart was dropped by City at the beginning of the month following a string of high-profle errors, with Manuel Pellegrini opting to give Costel Pantilimon a run in the first team.
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"Joe has to understand he is accountable and his performance level has to step up, but he will do that – there is no question about it," Rosler told BBC Radio 5 Live.
"Joe Hart is undoubtedly the best goalkeeper in England and he has shown that in the past.
"Manchester City should be very privileged to have England's number one. I can't see anyone really challenging him.
"He has enough time now to get his form sorted before the World Cup and I think it is a great opportunity for him [against Germany] because he will be tested."
Rosler also believes Germany will be under significant pressure in Brazil next summer as they bid to claim a first major title in 18 years.
He added: "Germany normally does very well in big tournaments. They've had a lot of praise for how they have played in the last few years and everyone in Germany is ready for a title.
"I think the pressure is on for the 2014 World Cup, even more than the previous ones because Germany have come very close, but haven't won anything since 1996 [when they claimed the European Championship]."