Hayden leaves Newcastle for Pompey on-loan for the remainder of the season.
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Blues boss John Mousinho said: “Isaac is a central midfielder who has a wealth of experience in the Premier League and Championship.
“He’s extremely comfortable with the ball at his feet and is also very strong defensively, possessing a lot of the attributes we were looking for.
“But he’s also a leader both on and off the pitch, so we’re delighted to be able to add him to the squad for the rest of the season.”