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Sir Alex Ferguson has praised his Arsenal counterpart Arsene Wenger for masterminding the recovery that sees the Gunners perched at the top of the Premiership.
Arsenal will open up a three point cushion at the top of the table if they defeat United, who are level on points, for the third time running in the Premier League.
Ferguson said he was not surprised by Arsenal's strong start to the campaign after their disappointing fourth place finish last term.
"I thought that Arsenal would make a strong challenge throughout the season and at this moment in time that has proved correct," said Ferguson.
"But it's a long season and other clubs will have a say yet.
"I know from my own experience that there are rough days ahead.
"I said in pre-season that there was no way that Arsenal would want to be 20 points behind Manchester United again.
"I expected them to be stronger."
Ferguson insisted Wenger did the right thing by allowing striker Thierry Henry to leave north London in the summer.
"Henry had served his time there," added Ferguson.
"Whether his departure has liberated Arsenal I don't know but Arsene did the correct thing by letting him go."
Written by majicka
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