The Blades have bolstered the defensive ranks on transfer deadline day with the loan signing of an international defender from the Bundesliga.
Greece defender Panos Retsos arrives at Bramall Lane from German outfit Bayer Leverkusen on an initial deal until the end of the season.
On the capture of a fifth addition in January, boss Chris Wilder commented: "Panos is another who comes to us with a great pedigree and hopefully we can help progress his career. He is young, talented and one that we've had our eye on for quite a while.
"He's an international who has played at the top level and cost a lot of money when moving across Europe. Now he wants to prove himself in the Premier League and this initial loan spells is a perfect opportunity for Panos to have a look at us and us to have a good look at him."
Retsos, 21, was born in Johannesburg to Greek parents and started his career with Olympiacos, for whom he made his debut for in the Europa League in August 2016.
In February 2017 Retsos became the youngest player to captain the famous Greek club, in a cup tie against Atromitos, aged 18 years, five months and 30 days.
An exceptional first professional season saw Retsos catch the eye of many European clubs and in the summer of 2017, the versatile defender joined Bundesliga outfit Bayer Leverkusen for a fee of nearly £16 million.
In Germany, Retsos has made 35 appearances, including six this season, scoring one goal, against RB Leipzig in April 2018.
On the international scene, Retsos opted to represent Greece, despite being eligible for South Africa, and he made his senior debut in August 2017 against Estonia in a World Cup qualifier.