Hartson, who played 54 games for the north Londoners, says Sanchez needs to put himself in the shop window.
“Remember, he’s playing for his next move,” Hartson told the Daily Star.
“The boy either sulks for four and a half months – although he doesn’t he seem like a sulker – or he gets over it.
“He knows he’s getting a move now, it’s set in stone, the ball is in his court.
“His contract is running down and he’s not going to sign a new one. He’s got to show a good attitude for his form and own mental state.