He is already 30 years old, and now his contract is remaining for the 3 more years mean this is prime time for him and if he moves to Newcastle United then surely this could be good for him otherwise he can face problems as well with his position in first eleven is also not clear right now.
And you really think that Maguire puts so much emphasis on playing in the starting that he will renounce for the salary he currently gets at United? Newcastle won't pay him 100% as much as ManU and if he gets his salary without playing he'll be happy with that too. Pure stupidity from ManU to conclude such a contract.