"It's huge," Ellinger said. "He's our attacking midfielder."
An eight-year Major League Soccer veteran, Williams injured his groin early in a 3-1 loss at the Los Angeles Galaxy last weekend, and he's expected to be out four weeks - which is actually less than the team initially expected.
Team spokesman Trey FitzGerald said Williams did not undergo an MRI exam Monday as planned, because trainers believe the injury isn't "as bad as originally feared."
In any case, RSL will look different against the Rapids than it has in its two road games so far.
Forward Clint Mathis will move back to replace Williams in the midfield, with midfielder Dipsy Selolwane moving up to play forward alongside Jason Kreis.
That might not be such a bad move, either, since Selolwane is a natural forward who seems inclined to play further upfield - he took the shot that Kreis turned into the first goal in team history against the Galaxy, after the goalkeeper deflected it - while Mathis has experience setting up teammates from the midfield.
"Clint's our guy," Ellinger said. "He's played there in preseason once or twice already, so we'll be fine with that."
Still, Ellinger is wary of playing without Williams, who was "one of our best players" in the scoreless, season-opening draw against the MetroStars in New Jersey but whose absence hurt against the Galaxy.
Once Williams left the game in the ninth minute, Ellinger said the team "didn't have that rhythm" it usually has among Williams, Kreis and Mathis, "which was one of our keys in the first game."
Veteran Chris Brown probably will enter the lineup in the right midfield spot that Selolwane has occupied, while rookie Luke Kreamalmeyer is expected to start on the left side, with Marlon Rojas moving from there to the back line to replace Matt Behnke.
Behnke was benched after the Galaxy's Landon Donovan sneaked past him to head in a decisive goal just before halftime.