The Manchester City keeper has frequently come under the spotlight this season, leading to questions over his place in the national side for the upcoming World Cup qualifiers against Montenegro and Poland.
However, Jagielka is in no doubt that the 26-year-old remains England's number one.
"There is no question in my mind how Joe will react and how he will perform," he told reporters. "He is a fantastic goalkeeper and a fantastic lad.
"If he does make a bad decision, the ball ends up in the back of the goal - he knows that is part of the territory for him.
"He knows he has the full backing of the whole squad and I am sure he is delighted to be here and he is looking forward to playing."
England play their penultimate World Cup qualifier on Friday when they welcome Montenegro to Wembley Stadium.