Vernon Davis active for 49ers

ST. LOUIS -- Vernon Davis is back to help the anemic San Francisco 49ers' offense Thursday night against the St. Louis Rams.

However, the San Francisco defense will be missing star inside linebacker Patrick Willis, who will not play because of a groin injury that required him to leave the team's loss to the Indianapolis Colts in the third quarter last Sunday. Michael Wilhoite will start in his place.

Willis warmed up but was deemed unable to play. It was believed all week that he'd have difficulty getting ready on a short week. If Willis has no setbacks, he should be ready to play against Houston on Oct. 5.

The 49ers' defense will be challenged with both Willis and star pass-rusher Aldon Smith out. Smith entered a treatment center for alcohol abuse this week. He is expected to be away from the team for at least a month.

Davis missed Sunday's game, his first since 2007. Davis practiced on a limited basis this week, but said he is not 100 percent, so there are risks. Hamstrings are easily aggravated. However, the 49ers' offense badly missed him against the Colts as it scored just seven points.

Also, as expected, No. 3 cornerback Nnamdi Asomugha will not play after hurting his knee Sunday. Rookie Darryl Morris was promoted from the practice squad this week and will play.

Two young 49ers -- running back LaMichael James and receiver Jon Baldwin -- will get a chance to provide a spark. They are both active for the first time this season. James had been out with a knee injury. Baldwin was a healthy scratch in his first three games with the 49ers after coming over from Kansas City in an August trade for fellow former first-round pick A.J. Jenkins.

The 49ers' other inactive players are quarterback B.J. Daniels, receiver Chris Harper, receiver Marlon Moore, fullback Owen Marecic and guard Joe Looney.

Meanwhile, for St. Louis, running back Isaiah Pead is among the inactives in something of a surprise move. With starter Daryl Richardson battling a foot injury, Pead was prominently involved in last week's game against Dallas but he didn't factor into Thursday's plans, apparently.

Pead's inactivity allows rookie running back Zac Stacy a chance to play and it seems likely he'll get touches to help spell Richardson. Of course, if Richardson aggravates his foot injury again, the Rams will be thin at running back with only Stacy and fellow running back Benny Cunningham having any carries under their belt.

St. Louis' other inactive players are center Barrett Jones, guard Brandon Washington, guard Mike Person, tackler Rodger Saffold, tight end Mike McNeill, and defensive end William Hayes.

ESPN.com's Rams reporter Nick Wagoner contributed to this report.