The 30-year-old was taken to hospital after suffering a seizure during the second half of the Blades' win at the Madejski Stadium on November 23.
Fleck sat out Sunday's 2-0 home victory over Bristol City - a match he was actually suspended for after receiving his fifth yellow card of the season against the Royals.
Blades boss Paul Heckingbottom delivered a positive update on the Scotland international in his pre-match press conference on Thursday. World Cup 2022
He said: "He's been in again today. He's got the all clear from the specialists so we're going to introduce him into training next week.
"He's been doing some running with (sports scientist) Lee McMahon and hopefully we'll have him back next week."
