Ferguson is keen to keep St Ledger, however, and has told the Bhoys they have "no chance" of landing the 25-year-old at the moment.
Speaking after the 2-0 defeat by Chelsea in the FA Cup, Ferguson said: "They have enquired but are a million miles away from getting Sean. The value was set when he went to Middlesbrough.
"I have had a couple of conversations with Sean and he is perfectly happy here. I definitely want to keep him. He is one of the best defenders in the league.
"I have no doubt he can play at a higher level but there is a valuation. If someone meets it I might have a different conversation with Sean. Celtic have no chance with what they are offering."