Celtic signed Ntcham from Manchester City last summer, and the 22-year-old has gone on to establish himself as one of Brendan Rodgers's key men at the Glasgow giants.
However, the club's Champions League exit to AEK Athens will lead to a drop in revenue, and clubs could now potentially begin to target some of Celtic's most highly-regarded players.
According to Goal.com, Porto have decided to make a bid of £13m for Ntcham, only to see it rejected by the Scottish champions.
The report claims that Porto are likely to return with a new offer, while the Frenchman is said to hold an interest in making the switch to Portugal.
Ntcham still has three years remaining on his contract.