Gallagher, currently on-loan with Crystal Palace, saw action in England's World Cup qualifying win against San Marino this week.
Cech wrote for chelseafc.com: "Conor was very close in terms of making the team here in the summer and we were really happy with him but it was the same as with Billy Gilmour and we felt at that stage of their careers they needed to go and play as many games as possible.
"They are both at the stage where we really want them to become core players for Chelsea so it was better for them to go and play.
"For Conor to get the England call-up was absolutely amazing and to make his debut was obviously even more amazing. If he didn't go to Crystal Palace and perform but rather stayed here and maybe didn't play every game then that would never have happened.
"Once you start to play regularly in the Premier League and show what you can do, that gives you all these chances to be selected. If you have the opportunity to do it when on loan, you can come back to Chelsea in a much better and stronger position.
"Conor will come back to us in a completely different situation than he was six months ago and that's the beauty of it. If everything works out perfectly, this is the idea behind it and obviously it's great to see that it's worked out perfectly for Conor so far."
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