Reid, 23, spent half of last season on loan at The Valley and will now move down a league permanently. The Blades signed the Londoner from West Ham last summer but he failed to impress and only made seven appearances. He did, however, play a part in United's 1:0 loss to Mansfield last Saturday at Field Mill.
He joins Athletic on a free but United have included a 'healthy' sell on clause.
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