The 22-year-old winger has been attracting interest from England, with Blackburn Rovers and Leeds United said to be keen on taking him to the Championship.
Mackay-Steven is, however, tied to a contract at Tannadice until 2015.
McNamara acknowledges that every player has his price, and that a sizeable bid could force his hand, but for now he is looking ahead with Mackay-Steven very much part of his plans.
He said: "It's part and parcel (given the) player's ability.
"Johnny Russell left there going into his last year, which we had to do. We couldn't afford not to. That's the nature of the business. The club has a selling point if players aren't going to sign new contracts.
"Fortunately Gary has two years to go so it puts us in a stronger position.
"Speculation means he is doing things properly and doing well, so long may it continue."
McNamara has been giving more attention to additions than departures of late and has hinted that the return of Mark Wilson to Tannadice may not be the last piece of transfer business he completes before the summer window closes.
He added: "I never say yes or no. Things change.
"Before Mark came in, we thought that was us with the transfers, but the opportunity came in. Whether that's the last, we'll wait and see when the window shuts."