The Eagles practiced Tuesday for the first time with a reshuffled offensive line that featured four players in different spots from the starting lineup in Sunday's game.

Left tackle Jason Peters was the only player at his original position. Todd Herremans slid from right guard to right tackle. David Molk played center in place of Jason Kelce, who underwent surgery for a sports hernia on Tuesday.

Matt Tobin, who missed the first three games, lined up at left guard with the top unit. Dennis Kelly, who had been the left guard, moved to the right side. That likely will be the lineup when the Eagles play the San Francisco 49ers on Sunday.

"Any business in the world, a new guy is going to have to step up at some point," Molk said. "If he doesn't, you're not going to trust him."

When NFL games started last fall, Molk usually took a nap. He lived in his native suburban Chicago and helped the Lemont High School football team. He was only two years removed from being named the top center in college football at Michigan, but Molk had no NFL contract and figured he'd need to wait a full season to get a chance.

That came when he signed with the Eagles in January. He earned a roster spot during the summer, and he is now in position to start the next six to eight weeks while Kelce recovers.

"This is why I'm here," Molk said. "This is why I played in college. This is why I worked my entire life. This is the opportunity."

Replacing Kelce is a difficult task. The Eagles offensive line went into a tailspin after Kelce's season-ending injury in 2012. Quarterback Nick Foles described Kelce as a "rock" on the line. At least Kelly and Tobin were on the roster last season and are familiar with the scheme and the coaching staff. Molk is more of an unknown.

A standout at Michigan, Molk won the Rimington Trophy, awarded to the nation's best center, for the 2011 season. He was a seventh-round pick by the San Diego Chargers, falling in part because of his 6-foot-1, 290-pound stature. He remained buried behind veteran Nick Hardwick.

Reserve centers usually need to play guard, as well. Molk was viewed purely as a center, which cost him a roster spot after a coaching change and limited his options on the free-agent market.

The Eagles targeted Molk in January as an under-the-radar signing. He was drawn to Philadelphia because the up-tempo offense was similar to the one he played in at Michigan.

"I had a better knowledge of what this is and it fit me better," Molk said. "If there was a place I was going to make it and do something with it, it's going to be here."

Molk's first action of the season came in Week 1 at guard after injuries to Evan Mathis and Allen Barbre. Playing center on Sunday was more natural for him. He knew it would be an extended opportunity after Mathis texted him the news about Kelce on Monday morning. Molk now has the benefit of knowing he will start.

"It's so much nicer when it's declared what your job is," Molk said. "I know exactly who we're playing, I know exactly what we're doing, I know exactly what my job is declared as going into the game. Thank you!"

Molk learned the system with the help of Kelce and former Eagles center Julian Vandervelde, who was waived in August after back surgery. Offensive coordinator Pat Shurmur said Molk is "already a good decision-maker," and coach Chip Kelly said the Eagles did not have communication issues after Molk entered the win over Washington.

Because of Molk's size, he is susceptible to being overpowered by bigger players. That happened at times against the Redskins. Shurmur, who was once an undersize college center, said that Molk's quickness and instincts can help and that he must maintain body control and take the proper angles.

Molk said he must highlight his strengths: quickness, agility, and good hands. If he doesn't, Molk said it "gets weird."

The 49ers could put pressure on Molk, although he pointed out that other reserves would be in the lineup. Tobin is likely making his first career start, too.

"If I succeed, everyone's going to trust me and everything's going to be great," Molk said. "I don't want us to get worse just because I'm in."