TEMPE, Ariz. (AP) _ If Emmitt Smith can't go on Sunday in Detroit, undrafted rookie Larry Croom will start at running back for the Arizona Cardinals.

The former UNLV back is humble about the opportunity.

It's definitely a blessing,'' Croom said.I didn't expect it, especially not this year. Coach Wilson (running backs coach Kirby Wilson) always told me to keep working hard and you never know what will happen.''

Coach Dennis Green had declared Smith out of Sunday's game with a sprained left big toe. Smith didn't practice Wednesday, but was listed as questionable, which is supposed to mean there's a 50-50 chance he can play Sunday.

He has an amazing ability to heal,'' Green said,so we'll see.''

Green called Smith one of the most resilient players he has coached.

Emmitt really goes hard at the rehab aspect of it,'' Green said.Some of the very best players seem to have a body type, a healing capability, that they can play with a lot of pain and can get back to the field very quickly.''

Croom has moved ahead of Josh Scobey on the depth chart to become Smith's backup after Troy Hambrick was lost for the season with a torn foot ligament.

He's a very quick player,'' Green said of Croom.His shiftiness is pretty obvious every time he touches the football. He's also very dedicated.''

Croom carried 10 times for only 25 yards in last week's 13-3 loss to the New York Jets. Arizona managed just 71 yards rushing in the game.

But Croom, who had played almost exclusively on special teams before Hambrick was hurt, said at least he got a taste of what it's like to carry the ball in the NFL.

There were some mistakes I made that I've got a chance to correct by looking at film,'' he said.If I didn't play, I wouldn't have a chance to make up for those mistakes.''

Croom started his collegiate career down the road at the University of Arizona, where he rushed for 257 yards and a touchdown as a redshirt freshman in 2000.

Arizona coach Dick Tomey was fired after that season, and Croom decided to transfer to UNLV after the following spring practice under coach John Mackovic. Even though he was listed as the No. 1 running back, Croom was splitting time with two others.

Coach Mackovic said the first day that he was throwing the ball 40 times a game,'' Croom said.I like catching the ball, but that's like two or three carries (a game).''

Croom joined coach John Robinson at UNLV, and after sitting out a season under NCAA rules, rushed for 605 yards as a junior and 932 yards as a senior.

Green likes players who work their way into the mix after not being drafted.

We think they come into the league with a very hungry approach,'' he said.Over the years, we've had a lot of success, both at San Francisco and Minnesota, with players like that.''

If Croom starts, he will be the fourth rookie in Arizona's offensive lineup. John Navarre will make his NFL debut at quarterback, taking snaps from rookie center Alex Stepanovich. The fourth rookie is wide receiver Larry Fitzgerald.

That's all you ask for sometimes is a chance,'' Stepanovich said.The way things have gone this year, a lot of rookies have had a chance, a lot of them have done well.''