The 26-year-old has been in lethal form for the Swans, scoring 16 goals in 24 appearances, with 12 coming in the Premier League, leaving him just one goal behind current leading scorers Robin van Persie of Manchester United and Liverpool's Luis Suarez.
The Welsh club have stated that the Spaniard, who moved to the club for £2 million during the summer from Rayo Vallecano, is not for sale.
|14/1||Michu is 14/1 with Bet365 to be Premier League top scorer
"It won't be easy for them to keep hold of him," said Coleman, who attended the FA Cup game in which Michu came off the bench to score against Arsenal after just 73 seconds.
"If any player at Swansea City is performing like Michu is then the big boys are going to look, and if they want you enough they will pay the money.
"As much as Swansea want to keep him, they won't be able to say no to that. They might say he is not for sale, but everyone is for sale at a price."
With big clubs such as Atletico Madrid declaring their interest in Michu, the Wales coach believes it is inevitable that a top team will make a move for the player, but concedes a January move seems unlikely.
"A lot of people will say he remains an unknown entity, but if he repeats what he is doing then that is when the danger will come," added Coleman.
"That is when one of the bigger teams will probably take a chance on him."