Danny Wilson has added to his striking options ahead of the new season with the signing of striker Chris Porter on an initial two-year contract.
Porter, 27, was a free agent after being released by Derby County at the end of last season, with his last goals coming against the Blades at Bramall Lane for the Rams second string, as they clinched the totesport.com league title in May.
The Wigan-born marksman also started for County in their 1-0 victory against United in the Championship in February.
Boss Wilson commented: "It is difficult trying to find strikers who will score goals at this level and if you look at Chris's career, possibly his most productive time was when he was at Oldham in this division where he scored a lot of goals. We know what he is about and he is very hungry after overcoming some injury problems.
"He is a terrific lad who will work his socks off for the cause. He is a good size, he heads the ball well and will score us goals. He is a good link-up player too and will play with different partners.
"We have given him a stringent medical to check he is OK. We have spoken to the boy and other medical people who have looked after him in the past and they are confident his problems are well behind him now."
After an impressive start to his career at Bury, Porter joined Oldham Athletic for £150,000 with the fee being decided by tribunal in 2005. The move proved to be a successful one and he finished the 2006-07 campaign with 22 goals to his name in League One as the Latics reached the play-offs.
This form alerted clubs in the SPL and Porter appeared set for a 'cross-border transfer' to Dundee United, before he turned them down in favour of a switch to Motherwell in July 2007.
His debut season proved to be a fantastic one, as he scored 15 goals to finish the club's top scorer and secure a joint third place finish in the SPL, as 'Well qualified for the 2008/09 UEFA Cup.
Another good start to the 2008/09 campaign, saw Porter linked with a return south of the border in January 2009 and the targetman seemed set to join Cardiff City only for Derby to hijack the proposed move to sign Porter for an undisclosed fee. In total he scored 27 goals in 65 appearances in Scotland.
The Lancastrian could make his United debut against his former employers at Boundary Park on the opening day of the season.
Porter joins less than 24 hours after the Blades snapped up Ryan Flynn for an undisclosed fee and committed Jordan Slew to a new contract.