Foster, 41, is charged with aggravated battery and aggravated sexual assault.
Foster was on parole for armed robbery when he allegedly attacked a 65-year-old man from suburban Schiller Park. The victim was found just after 12 a.m. Saturday near 8th Street and Michigan. He was undressed and dazed from his injuries.
The attack has people who live and work around Chicago's South Loop concerned.
"It is kind of shocking to hear that something like that would happen in this area, you know? It makes you just kind of wonder, what's really going on with our city?" said Beryl Armstrong, who works in the South Loop.
Foster was paroled last May after serving part of a 20-year prison sentence for armed robbery.