Gallagher chose to play for England this week after spending his junior career with Scotland's youth teams.
Blake said: "Chambers is in the first team at Saints at the moment and is doing very very well. He can [play at centre back] all the time, but obviously that's not my decision.
"He's played wide right and centre midfield and always turned out accomplished performances.
"I personally think long term that Calum will make an excellent centre half.
"He's doing very well at right back but he's got very good reading of the game and is very clever.
"The reason why I selected him as a centre back is because I know what he's like getting back and forward in the right back position.
"I brought him off at half-time (Saints colleague Harrison Reed coming on in his place for his England under-19 debut) because I know he has a game on Saturday."
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