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A secondary thinned by injuries could be of primary importance for the Chicago Bears this week as the slumping defending NFC champs prepare for a Detroit Lions' offense that makes no pretense about its desire to put the ball in the air.

In fact, the switch at quarterback, where backup Brian Griese will replace ineffective Rex Grossman, is only part of the shuffle that coach Lovie Smith is set to enact. The changes in the Bears' secondary, however, aren't a matter of choice, but necessity, as early-season attrition is severely testing the unit's depth.

Right cornerback Nathan Vasher will miss Sunday's game at Detroit, and possibly as long as a month, with hip and groin injuries sustained in last week's loss to the Dallas Cowboys. And his opposite cornerback, Charles Tillman, could miss Sunday's game with a sprained right ankle.

With free safety Mike Brown already on injured reserve and out for the year with a torn anterior cruciate ligament suffered in the season-opener, strong safety Adam Archuleta could be the lone starter in the lineup Sunday afternoon. And Archuleta, whose liabilities in pass coverage were again exposed in the Dallas game as Cowboys' quarterback Tony Romo seemed to zero in on him, will be playing with a cast on his broken right hand.

"Guys are definitely going to have to step up now," said nickel cornerback Ricky Manning, who likely will start in Vasher's spot. "We talked a lot about having good depth on our side of the ball. Now we're going to find out."

If Tillman can't play, he probably will be replaced by rookie Trumaine McBride, a seventh-round draft pick who would make his first regular-season start. Danieal Manning already has replaced Brown at free safety. Reserve safety Brandon McGowan may assume the nickel defensive back role.

There is, of course, no good time for the kind of rash of injuries the Bears have endured in their secondary, but the fact Sunday's opponent is the Lions only exacerbates the situation. Under pass-happy offensive coordinator Mike Martz, the Lions have one of the most skewed attacks in the league, with nearly three-quarters of their 207 snaps in three games having been pass plays. Detroit leads the NFL in average passing yards per game (345.3), attempts (138), dropbacks (153) and completions (91). On the flip side, the Lions rank last in rushing attempts (54).

If there is a possible bright spot for Chicago, it is that injuries to two defensive front seven standouts, tackle Tommie Harris (knee) and weakside linebacker Lance Briggs (groin), don't appear as serious as originally thought. The availability of both those players may not be determined until later in the week.

Len Pasquarelli is a senior NFL writer for ESPN.com.