The Manchester City midfielder controversially started Friday's crucial 4-2 defeat to Romania, which dumped the Young Lions out of Euro 2019, on the bench.
He came on in the second half - having scored a brilliant solo goal in Tuesday's loss to France - but could not rescue the Young Lions.
Foden started just 14 games for City last season and Under-21s boss Boothroyd believes he is coming to a crucial period of his development.
He said: "I think his next six months, three months will be an indicator of where he gets to and what he is ready for internationally. But, right now, September-time, I'd imagine he will be with me.
"He is the hardest player to get off the training pitch that we have got. I literally have to chase him around to get the ball off him and get him in.
"Because he has that youthful exuberance where he wants to play every second, every minute of every game.
"When I looked at him before the (Romania) game, when I picked the team, I was thinking 'How can we get the best out of this kid in the next five games?'
"That was the aim, the goal, what we wanted to do. Part of that was, 'OK, we will get him on the pitch as much as possible' but I have to use my eyes, my experience and look at players.
"I need to think 'Right, OK, he needs to come off now, it looks like he is blowing, he's leggy. Give him a bit of time, let the game settle down and put him on."'
Boothroyd also outlined why he left Foden on the bench for the must-win clash in Cesena.
"Well, you do (want to start him) but if your best player has a really strong chance of getting injured because of the work that he has done, then no - you don't," he said.
"It's all right trying to win the next one but it is a balancing act. I think the fact that Ses (Ryan Sessegnon) pulled his hamstring is a testament to the fine line that you walk."
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