GREEN BAY—At first blush, it's as though a player like Darren Sproles defines the term "X factor."

Philadelphia's veteran running back and return dynamo is one of the smallest players in the NFL at 5-6, 190.

He's not his team's primary ball-carrier, yet, he has four rushing touchdowns.

He's also not the primary pass-catcher, yet, he averages a robust 12.2 yards per reception.

Now in his 10th year and with his third team, he no longer returns kickoffs, but he leads the league this season with a 17.0-yard average on punt returns, with two long TDs.

Then again, to call him an "X factor" unfairly undersells Sproles' impact on games. That impact is not only pivotal, it's constant – constant series after series because he can handle the ball in different ways, and constant week after week because he never stops doing his thing.

"The guy's a game-changer," Packers cornerback Tramon Williams said.

The Packers know all about Sproles and his elusive combination of speed and shiftiness. Sunday will mark his third visit to Lambeau Field in the past four seasons.

He lit up the building in 2011 with the Saints, catching seven passes for 75 yards, returning a punt 72 yards for a score and also returning a kickoff 57 yards. The following year, he came back and caught five passes for 44 yards and a TD and returned a kickoff 35 yards.

"Darren Sproles has done it for a long time in this league," Head Coach Mike McCarthy said. "Four-down player, in my opinion. He's definitely a focus point for us.

"He's so quick and explosive, I think sometimes people forget how smart a player he is. I've always found him to be very instinctive. People have been creative with him."

Unfortunately, creativity doesn't help in trying to neutralize him. His fit in Chip Kelly's offense is almost too perfect. He makes defensive coordinators "matchup conscious," and the hope is not to get caught in a bad matchup if he's lined up in the slot or slipping out of the backfield.

Even with the best matchup, he's no easy mark in the open field, and despite being traded to Philly for a fifth-round pick this past offseason, he hasn't lost a step. His 49-yard TD run vs. Jacksonville was a career long. His 57-yard reception vs. Indianapolis was his longest since 2010 while with San Diego.

"You go back and look at what he does to teams and you wonder how these different teams let him go," Williams said. "You have to know where he is at all times."

In the return game this past Monday night vs. Carolina, Sproles fielded a down-the-middle mistake of a punt and returned it 65 yards for a TD, his second this year following an 82-yarder vs. San Francisco.

McCarthy noted the return game reaches "a premium" when the weather turns cold in Green Bay. Packers punter Tim Masthay said "there are no secrets," only two ways to deal with a returner like Sproles – great placement and great hang time with "manageable" distance.

"Even better is if you can combine the two," Masthay said. "The trick is that's extremely difficult to execute, particularly if the conditions aren't great. We have to find a way to execute in spite of the conditions."

Winning what qualifies as the biggest game of the season to date may depend on it.

With the Eagles currently sitting in first place in the NFC East at 7-2, Packers QB Aaron Rodgers acknowledged this week the result could factor into potential playoff seeds. He also called it the kind of game teams have to win at home if they want to make a postseason push.

Moreover, the only teams the Packers have played thus far in 2014 that would be in the playoffs had the season ended after Week 10 – the Seahawks, Lions and Saints – represent Green Bay's three losses. Those three were on the road. This one's at home, where the Packers have put together their best performances.

"It's going to allow us to see where we're at," receiver Jordy Nelson said. "I think our tougher games, we haven't played our greatest, so we need to do that."

Particularly against the "X factor," or "game-changer," whichever fits Sproles better.

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