The ex-Celtic star took to Twitter shortly after the 2-0 defeat in Luxembourg, where hosts Progres Niederkorn advanced to the second qualifying round of the Europa League at the expense of Rangers.
He wrote: “Totally embarrassing from Rangers! Maybe concentrating on what goes on on the football pitch rather than boot colours would be wise...”
His comment was in response to Rangers boss Caixinha deciding to ban his players from wearing green boots because the colour is associated with rivals Celtic.
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