The 29-year-old midfielder returned to training after a broken leg he suffered in March.
But Wagner does not want fans to expect too much. Williams still has a long way to go before he can make a meaningful impact on the season.
"Danny Williams was in the squad even if he only trained a week, which is something I usually would not do," said Wagner to The Huddersfield Daily Examiner.
"But after Hoggy said he wasn't able to play and Aaron Mooy left us very late, I made the decision Danny should be in the squad as we might need him.
"Happily we didn't need to use him because I think he needs some further minutes, but it is always good to have him in our group because he is a very positive character."