The centre-half has impressed for Celtic under Ange Postecoglou since arriving from the English Premier League outfit and the Australian manager wants to keep him in Glasgow beyond the end of the season.
Celtic have an option-to-buy clause for the 24-year-old, who has been capped eight times by the United States, but it is being claimed that the fee is £6million.
Sky Sports News are reporting that Celtic and Spurs are negotiating over the payment plan of the deal, with the cinch Premiership side unlikely to want to shell out in full for the player straight away.
Celtic are also hoping to conclude a deal for fellow loanee Jota from Benfica. They also have an purchase agreement for the Portuguese winger, believed to be £6.5m.