The Blades blew a gilt-edged opportunity to put pressure on the top six by losing at Birmingham City.
Despite taking an early lead through Mark Duffy, United were well beaten to slip to 11th – their lowest standing since mid-August – and three points adrift of Millwall in the final play-off place with just two games remaining.
Wilder was hugely disappointed in his side’s display once pegged back on the half-hour, admitting he has to work out if such a lifeless effort was down to fatigue or a poor attitude.
“I can’t build sides like I have done in the past with that sort of mentality and attitude,” said the 50-year-old. “We have to play as a team and stick to a plan, which we have done for the majority of the season.
“You sacrifice yourself to make the team get the best result. I will be looking pretty closely now at certain players.
“It is hugely important that we finish the season off on the front foot – and to finish as high as possible. We have an obligation to our supporters.
“There might be a few changes next week (at home to Preston), a few surprises. I will play a team that has a big hunger and desire.
“Where I have to be careful is I don’t get it wrong, in that people are tired physically and mentally rather than people feeling comfy with an attitude that says, ‘I am all right, I am a Championship player now’.
“I have to make that decision. If I feel that has happened then they are not for me. But if it is just a case of running the race and having given everything then so be it.
“Jack O’Connell is a prime example. He is out on his feet, but I wanted to keep him on the pitch. When people talk about player of the year awards, he has not missed a game or minute and has been outstanding.
“I have to make sure I get my judgment of players right.”
Wilder had strong words in the dressing room at St Andrews after goals from Marc Roberts and Jacques Maghoma sealed a 2-1 win for the Blues.
He was critical of scorer Duffy, the former Birmingham man who was booked for celebrating provocatively in front of the home supporters.
“Players should self-police,” added Wilder. “Duffy is someone I had a pop at. What he did after scoring was poor. To get the supporters on his back was unnecessary.
“He had done the hard bit by scoring. He didn’t need to rev it up and get booked.
“He should get back to the halfway line and on with the game – go and stick the ball in the net again. He doesn’t need to antagonise anyone. That upset and annoyed me.”