The 24-year-old USA international has missed the last seven matches after suffering a foot injury in September.
He could be in contention to return to the squad when promotion rivals QPR visit the Madejski on Saturday.
"It felt really good to be back," Williams told the club website.
"This is the kind of injury that takes a long time to heal completely but I feel like I'm back now. I feel like I'm ready to go.
"I have to see how the sessions go this week, that I don't have too much pain. It's the gaffer's decision but I'll tell him I'm ready and he can trust that I will put everything in."
- Sports & Recreation