United have signed Northern Ireland international Daniel Lafferty on a season-long loan.
The Burnley left-back is United's 12th signing of the summer, the second of transfer deadline day and the third in 24 hours.
Blades chief Chris Wilder commented: "We are trying to create genuine competition for places and with Kieran Wallace going out today, Daniel will challenge Chris Hussey for the left-back role, as will Ethan, with the central defenders.
"Daniel is a good age and has shown a great appetite to join us."
27-year-old Lafferty began his career with Celtic and spent time on-loan with Ayr United, whilst in Scotland.
In 2010, Lafferty moved to Derry City and spent two seasons in Ireland, prior to making the move to Burnley, then in the Championship.
Capped 13 times by his country, Lafferty has made approaching 50 appearances for the Clarets since January 2012, although in more recent times he has enjoyed successful loan stints with Rotherham United in the Championship and two spells with Oldham Athletic in League One.
Lafferty joins Ethan Ebanks-Landell and Caolan Lavery as newcomers at Bramall Lane. All three could make their Blades debuts in the televised League One clash at Gillingham on Sunday.