John Fleck's second-half double against Aston Villa earned the Blades back-to-back top-flight wins for the first time since 2006. Jack Grealish missed a Villa penalty to end any chance of getting back into the game.
Smith said: "Our overall performance wasn't what I wanted from this group of players. We didn't cause them enough problems.
"The second goal was a bit of a killer but they probably deserved the win on the day.
"The players need lifting, we've lost three games on the spin but I just said to the players that a few games ago we were walking off the pitch at Old Trafford a couple of days after beating Newcastle and we should have got a win at Manchester United."