Rachal, who signed a one-year deal with the Bears on April 23, started 38 games in four season with the San Francisco 49ers.
"I need to just seize the moment and take advantage of it," Rachal said Monday. "I had a good dude before me (Spencer), but now that I'm in and have the opportunity. I want to just maximize it. I was a little (surprised), but I've always been a hard worker. I show up to work every day and let that take care of itself.
"The team called my number and that's what I'm here for, to help anyway I can."
Spencer joined the Bears last summer after spending his first six NFL seasons with the Seattle Seahawks. Although he started mainly at center during his time in Seattle, Spencer played well his first season in Chicago at right guard (14 starts) before he moved to left guard in the offseason.
But Spencer struggled at times in the preseason and in the Bears' Week 2 loss at Green Bay.
"I guess they have to do what's best for the team, and I have to keep working and get better," Spencer said. "I'm a veteran, and I've been around a long time. I understand the process. All I can do is come out here and focus on what I have to do."