The 17 year-old left-back is being chased by the Premier League's biggest clubs this month.
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It was reported earlier this week that Chelsea were in the lead for Alao's signature.
However, the Manchester Evening News is reporting United have now made a formal offer for Alao. The terms haven't been disclosed, but the bid is now lodged.
Despite his youth, Alao made his first team debut for the Owls last month.