Johnson made 84 league outings for Portsmouth after originally arriving on loan from Chelsea in 2006, netting four times, including the spectacular long-range volley at home to Hull City in 2008/09 that was named goal of the season.
His impressive form for the Fratton Park outfit led to England honours under Fabio Capello before his big-money move to Anfield, after rejecting the opportunity to re-join Chelsea, who had also had a bid accepted by Portsmouth.
However, Sky Sports pundit Merson, who made more than 300 league outings for the Gunners from 1985-97, believes the Merseysiders are mad to have shelled out such a large fee for a defender.
“The Premier League is not blessed with 15 world-class left wingers so I don't see how a right-back can affect a game that much," he said.
"I've got nothing against the player, I think he was outstanding for Portsmouth last year, but I just don't think you should be spending that amount of money on somebody who plays at right-back.”
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