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Monk only took charge at Middlesbrough in June, but the former Swansea and Leeds boss was unable to mastermind the expected promotion challenge.
Middlesbrough, who won 10 of their 23 league games under Monk, are ninth in the second-tier table, three points behind Aston Villa, who occupy the final play-off place.
“Middlesbrough Football Club have parted company with manager Garry Monk,” the club announced in a statement following the come-from-behind win in Sheffield.
Monk managed Swansea in the Premier League between 2014 and 2015 before helping Leeds to a seventh-placed finish in the Championship last term.
He was unable to agree an extension to his stay at Leeds under new owner Andrea Radrizzani, leaving last May before swiftly taking up the reigns at Boro.
Middlesbrough had hoped to bid for automatic promotion under Monk, having been relegated from the Premier League in May.
But they are far adrift of that target and have reacted by relieving Monk of his position two days before Christmas.
“The club would like to thank Garry for all his hard work and dedication, and wish him all the best for the future,” the statement said.
“The club’s academy manager Craig Liddle will take temporary charge of first team affairs during the interim period while a successor is appointed.”
Middlesbrough’s first match without Monk is at home to Bolton on Tuesday.
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