The striker, who celebrated his 34th birthday last week, scored in victory over Bournemouth yesterday.
"He hasn't played as much as he would have liked. Off the pitch, he's been a fantastic leader," said the Blades boss.
"It's easy when you're playing and scoring, and Billy has scored so many goals for us, to carry out those leadership duties and when you're not it's a little bit more difficult.
"When you sign players, or keep people at the club, you're looking to sign and keep good people as well. You want ambitious players. You want ones who can overcome disappointment and are selfless. You want ones who can face up to challenges.
"Billy had offers to leave in January but he wanted to stay. It's been a different type of test, not that I want to put him through a test, but it is, and he showed his reaction. It speaks volumes."
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