The former Sheffield United and Rotherham midfielder has attracted interest from a number of clubs, including Rangers, and McCall has admitted defeat in his bid to extend his two-year deal.
McCall said: "Nicky Law will move on. I didn't think I would be able to get him to come to Scotland because he had really good offers financially in England but he came. We have had offers and we rejected them.
"He has loved his football and loved his time. When Nicky moves on he will go with our best wishes because he has been a joy to coach and given great moments to the supporters."
Law will join Darren Randolph in departing Fir Park, with the goalkeeper expected to sign for Birmingham.
McCall said: "He has not signed a pre-contract at Birmingham. I know there is interest there, as there is from a lot of clubs.
"He has been an outstanding servant and he certainly deserves his move. We will all miss Darren and he will go with everyone's best wishes because he is a model pro."