The 29-year-old revealed he fractured two bones in his back while on international duty, falling awkwardly during the 1-0 victory against Slovakia in the Nations League.
The former Rangers youth was out for six weeks before playing again for the Blades in their Premier League loss to West Ham on November 22.
Fleck revealed the extent of the injury on Monday ahead of United's home clash with 19th-placed West Brom, who sit four points ahead of Chris Wilder's side in the table.
"I fractured two bones in my back so it took some time to recover, but I can't use that as an excuse as I haven't played as well as I could," said Fleck.
"I don't know why that is and if I knew I'd change it. But I feel I'm getting better and the whole team is as well."
Wilder's side are bottom of the table and 13 points from safety but Fleck insists they have not given up hope of pulling off the greatest of escapes.
"I'm enjoying my football again but I'm not enjoying the position we're in, but we will keep working hard," he said.
"Staying up would surpass everything else. When we first came up the main aim was to stay up and the same again this year. We need to start winning some games to give us half a chance of doing that.
"I've seen that (the Derby points total) but it's not something I've thought about. I've always believed we'd get past that margin and our main aim is to stay in the league, not catch other people's points totals.
"Both teams have to win (on Tuesday) to give half a chance of staying in the league and we will go into the game with that mindset of we have to win it.
"It's not nice being stuck at the bottom for so long but that's down to us to put it right. We will keep working hard and strange things can happen in football, so you never know."