Denver, CO -- (Sports Network) - Broncos quarterback Kyle Orton sat out practice again on Thursday with his severely sprained left ankle and looks increasingly doubtful for Denver's Week 11 showdown with San Diego.

Orton was injured late in the second quarter of last Sunday's loss to Washington and did not return. Chris Simms relieved the starter and completed just 3-of-13 passes for 13 yards with an interception, but is in line to make his first NFL start in over three years.

The last time Simms started under center was Week 3 of the 2006 season when he suffered internal bleeding and had to have emergency surgery to have his spleen removed following a nasty hit. The injury cost him the rest of that season and all of the next.

Meanwhile, wide receiver Brandon Marshall (neck) was a full participant at practice Thursday after sitting Wednesday. Starting right tackle Ryan Harris (toe) has not practiced all week and is in danger of missing his third straight game. Running back Correll Buckhalter (knee) sat Thursday, one day after being limited.

Denver started the season 6-0, but has since dropped three straight to fall into a tie with the Chargers for first place in the AFC West.