On September 1, the Giants traded running back Ryan Grant to the Green Bay Packers for a sixth-round draft pick. At the time, the trade — involving a player who had never appeared in a regular-season game — seemed utterly inconsequential.

But on Sunday, that trade may come back to haunt the Giants.

Grant has emerged from the oblivion of the bottom of two NFL depth charts to become one of the best running backs in football this season. On Saturday, Grant shook off two early fumbles and ran for 201 yards in the Packers’ victory over the Seattle Seahawks. And now, as the Giants prepare to face the Packers in the NFC Championship game, stopping Grant will be every bit as important to their defensive game plan as stopping Packers quarterback Brett Favre.

No one can blame Giants general manager Jerry Reese for the decision to trade Grant. The Giants entered the regular season so loaded at running back that if they hadn’t been able to acquire something for Grant in a trade, they probably would have released him. Grant played well for the Giants during the preseason, running 18 times for 90 yards, but even if he had gained twice as many yards in exhibition games, he wouldn’t have been able to beat out the Giants’ other running backs for playing time.

The running back talent on the Giants’ roster is incredible: Brandon Jacobs had a 1,000-yard season despite missing five games with injuries. Derrick Ward was the Giants’ primary ball carrier six times, and he averaged more than 90 yards in those six games. Ahmad Bradshaw has burst onto the scene in December and January after filling in for Ward. Reuben Droughns has two different 1,200-yard seasons on his NFL résumé, but he’s hardly even been able to get on the field this year. With those four players in the mix, it’s not hard to see why Reese figured trading Grant was the right move.

In short, Grant was one of five different running backs on the Giants’ preseason roster who could have been starters on some other NFL teams. Not bad for a team whose supposed top concern was replacing Tiki Barber.

The 25-year-old Grant has had a tough journey from high school stardom at Don Bosco Prep in Ramsey, N.J., to setting the Packers’ all-time playoff rushing record. Grant enrolled at Notre Dame in 2001 and became the starting running back as a sophomore in 2002. But in his junior and senior seasons he fell behind Julius Jones, now of the Dallas Cowboys, and Darius Walker, now of the Houston Texans, on the Notre Dame depth chart, and he wasn’t chosen in the 2005 NFL draft.

The Giants invited Grant to training camp as an undrafted free agent and kept him on their practice squad in 2005. In 2006 he hoped to make the active roster, but he ended up spending the year on the non-football injury list when he suffered severe cuts to tendons and ligaments in his left arm after falling on a glass table. The 2007 preseason was his last chance to prove himself, and he did that well enough to convince the Packers to trade for him.

That wasn’t the end of it for Grant, though. Early in the season he rarely played because Packers coach Mike McCarthy didn’t think he knew all of his assignments in pass protection, and McCarthy worried that having Grant as his starter would leave Favre exposed on blitzes. That’s why Grant was barely a part of the Packers’ offense when the Giants played the Packers in the second week of the season. He did make one solid play in that game, catching a short pass from Favre and turning it into a 21-yard gain, but that was the only time he touched the ball. Just as he had been buried on the depth chart with the Giants, Grant was a fourth-stringer on the Packers, stuck behind running backs De-Shawn Wynn, Brandon Jackson, and Vernand Morency for the first six weeks of the season.

But Wynn, Jackson, and Morency couldn’t get the ground game going, and the Packers didn’t have a 100-yard runner in any of those first six games. So after the Packers’ Week 7 bye, McCarthy decided that the speedy, 6-foot-1-inch, 224-pound Grant could give the running game a spark. Grant did just that, picking up 100 or more yards in five of the Packers’ final 10 games.

And then came that 201-yard game against Seattle. Some coaches would have benched Grant after his two early fumbles, but McCarthy said afterward that he never considered that — Grant is his running back, and McCarthy has faith in him.

It took a long time, but Grant is finally a firmly established starter. Three months before the 2008 NFL draft, the Packers have already gotten more production out of their sixth-round pick than they ever could have imagined. The Giants are just hoping the trade that got them that extra draft pick won’t keep them out of the Super Bowl.

Mr. Smith is a writer for FootballOutsiders.com.