The veteran defender has resisted interest from Atletico Madrid this week, despite being relegated to a backup role this season.
Nadal has praised Pique's attitude, telling Mundo Deportivo: "What happens with Piqué is not only a sporting issue but other circumstances have influenced him, but he is still a very reliable player.
"The important thing is that you have motivation and ambition. And that is very positive. It is important in a squad to have players like that. He has self-esteem and, furthermore, nobody can argue with Piqué or put a but to his sports career."