LAKE FOREST, Ill. -- The Chicago Bears officially ruled Brian Urlacher out of Sunday's game against the Minnesota Vikings with a hamstring injury.

Although the Bears hope he can return before the end of the regular season, there's no guarantee.

"Exactly when he's coming back, I don't know, but he's coming back at some time, though," Bears coach Lovie Smith said.

In an interview Tuesday with FOX Chicago, Urlacher said doctors told him he could be out "two to four weeks, three, could be four." The Chicago Tribune, citing sources, reported earlier in the day that he was expected to miss three games and possibly the rest of the regular season because of a Grade 2 right hamstring strain.

"He's one of the leaders," Smith said. "He makes all of the calls. Just him being in the huddle helps. We're going to miss him. I can't belittle that at all. We're missing a great player. But you just have to look at Plan B. Plan B for us is pretty good if you do lose a guy like that."

Nick Roach will move to middle linebacker to take Urlacher's place, and Geno Hayes will start for Roach on the strong side against the Vikings.

"We have guys in position," Hayes said. "Nick is going to be a great guy. He can step in to that role and make those plays that we need to make at middle linebacker, so we're not really worried about that. We're going to still have Urlacher on the sideline giving us tips and things of that nature, so we're not too worried. We're going to do what we came to do."

The Bears also signed linebacker Dom DeCicco to a two-year deal Tuesday to add depth at the position. He appeared in all 16 games last season for the Bears, primarily playing special teams.

Urlacher, whose contract expires after this season, came up lame in coverage on the second-to-last snap of Sunday's overtime loss to Seattle. When asked if he expects to be ready for the postseason, provided the Bears make it, the eight-time Pro Bowler said "100 percent, yes."

"The sooner we get him back, the better," quarterback Jay Cutler said Wednesday. "But guys are going to have to step up as other guys have ... and we're just going to have to find a way."

The Bears welcomed back to practice receivers Alshon Jeffery and Devin Hester and guard Chris Spencer on a limited basis Wednesday.

Jeffery sat out the past two games after undergoing arthroscopic surgery on his right knee on Nov. 21 to fix an injury that occurred in the Bears' 32-7 loss to the San Francisco 49ers. The rookie will be asked to help relieve some pressure off No. 1 wide receiver Brandon Marshall, one of the most targeted wideouts in the league.