Roberts is this season on-loan at Celtic.
Boothroyd said: “I keep in contact with all the boys. We have a long squad of players that could play from the 1997 and '96 groups.
“We've got some massive talent and Pat will be a big player for us in the future - I'm sure of that.
“I just felt that Jack Harrison had warranted a place in the squad for the simple reason he had more assists and had been scoring more goals and is in better form.
“Pat is doing well at Celtic and played really well in the Champions League but I felt Jack had more game time."
He added: “I found Pat at the last camp and told him why he was not in the squad, and Izzy Brown is the same, another good player playing in the Premier League.
“But he hasn't got the game time yet to get into the squad, which is a good place for me to be. Because I know in two years time that when we hopefully get to Italy, all being well, we will lose some players through injury or other things, so it's important to have a good pool of players."