The 20-year-old former Manchester United trainee, a free signing following his release from Old Trafford, made his first appearance for the Blades at full-back in last week's home draw with Colchester.
"Marcus (Williams) didn't feature against Colchester United because Sean McGinty deserved a chance in the first-team environment," said Weir.
"Sean has done very well in training and it's important that the lads understand that if they perform during the week they will be given an opportunity on a Saturday.
"He did very well, athletically the lad is fantastic and will only improve. Obviously there is room for improvement like there is with all the players.
"For a debut at Bramall Lane in front of big crowd he really impressed me."