The 19-year-old was widely tipped to be included in the squad that Ireland manager Martin O'Neill named on Tuesday afternoon, but his name was conspicuous by its absence in the 33-strong list when it was eventually made public.
The Birmingham-born midfielder can also play for England but has already represented Republic of Ireland at Under-21 level.
Speaking at a press conference in Dublin, O'Neill said: "I spoke to both Jack and his father, and I think Jack just didn't want to make his mind up at this stage
He wanted to complete the season at Aston Villa - there are some important games for them coming up - and he wants to make his decision at some time in the future."
The Republic and England have been involved in a lengthy tug-of-love for Grealish for some time with O'Neill's number two trying his best to persuade the teenager to declare for Ireland during his spell at Villa Park as Paul Lambert's assistant.
He had indicated that he would not make a decision until the start of next season, and later hinted he might opt to wear the green shirt, but initially for Noel King's Under-21s side.
However, given his rise to the fore at Villa in recent weeks - he turned in a fine individual display in the club's FA Cup semi-final victory over Liverpool - O'Neill chanced his arm once again.
Asked if he was disappointed by the player's response, he said: "Yes, I suppose I am
He has been doing very well recently and it would have been nice, but that's Jack's decision.
"I spoke to the father and said I would like to have brought him into it, but of course you don't want to force someone who might not want to make that decision just at this moment."
Source : PA
Source: PA