Barry was ruled out of the tournament in Poland and Ukraine after suffering a tear to his lower abdomen in the 1-0 friendly win against Norway over the weekend, with Jagielka the man to come in.
The Everton centre-back played 90 minutes in Oslo as England kept a clean sheet and was thrilled when he was given the nod to join up with Euro 2012 squad.
"I was obviously a little bit frustrated and disappointed not to be in the original 23 but the manager was very honest and straight up with me," Jagielka said in an interview on Sky Sports News.
"He said: `Would you come and join up and be on standby and play a part in the Norway game and if anything does happen you'll be the going to the Euros'.
"As it is, I got to play 90 minutes for my country which was fantastic and a few days later I managed to make the 23."