The Manchester City keeper has faced criticism for a number of blunders in goal at club level, most notably for his performance in his side's 3-1 defeat to Bayern Munich in the Champions League last week.
"One week you can make the most unbelievable saves and the next you can make one mistake, and it's the mistake everyone will be talking about," Forster told Celtic View.
"It comes with the job so I feel sympathy for him. He'll pick himself up and move on, come back stronger and prove he can do it again."
Hart is expected to start in England's World Cup qualifier against Montenegro on Friday.