Quarterbacks Jason Campbell (knee) and Kyle Boller split reps with the first-team offense on Thursday for the second straight day. Regular starter Bruce Gradkowski (shoulder) worked on individual drills, coach Tom Cable said.
The coach added that he still doesn't have a firm grasp on which of the three will emerge as the starter for Sunday's game against the Denver Broncos. Right now, Boller is the only one healthy enough to start, Cable said.
The fact that McClain hasn't forced any fumbles or intercepted any passes so far is surprising, Marshall said.
"Yeah, I kind of point that out to him every once in a while," Marshall said. "First-rounder, you got to be an impact (guy), right? That's one of the ways you can impact" a game.
McClain was unavailable for comment. He said Monday that "99 percent" of the time he doesn't feel like speaking with the media.
With Marshall in the lineup the past three seasons, the Broncos featured an obvious go-to receiver, especially on third-down and on crucial plays. He surpassed 100 receptions each year.
"Now, there is not a focus," Asomugha said. "They have so many weapons that Kyle (Orton) is throwing the ball to everybody. They spread it around much more than they did in the past."
Orton is on pace to pass for an NFL single-season record 5,178 yards. Wide receivers Brandon Lloyd, Jabar Gaffney and Eddie Royal have combined for 103 receptions for 1,438 yards and six touchdowns through six games.
By comparison, Oakland's top three wide receivers -- Louis Murphy, Darrius Heyward-Bey and Johnnie Lee Higgins -- have 36 catches for 509 yards and one TD.
With Zach Miller the only other tight end on the 53-man roster, Cable is mulling over how to proceed. Options include using regular fullback Marcel Reece, promoting tight end Kevin Brock from the practice squad or going to a backup offensive lineman in obvious run situations.