Richards, who spent the second half of the 2007-8 season on loan with the U’s, joins on a month’s loan from Burton Albion and goes straight into the squad to face the Blades.
This was one of three pieces of good news for Wilder yesterday.
Midfielder Lewis Montrose signed on loan from Gillingham until the end of the season.
And Josh Parker, who has made three substitute appearances since signing non-contract terms with the U’s before Christmas, is extending his stay with United.
The former QPR youngster can play up front or out wide and has agreed a deal to stay until the end of the season.
Richards will be a familiar face to many U's fans after his previous loan spell.
With James Constable suspended for four games and Deane Smalley and Jon-Paul Pittman both on the long-term injured list, United have turned to the experienced forward. Now aged 32, the powerfulstriker began his career at West Brom and has played more than 400 senior games while scoring more than 100 goals.
Wilder welcomed Richards’s arrival. He said: “Justin is a strong lad who scores his share of goals and also holds the ball up well and brings other people into the game.
“It is an extraordinary situation with suspensions and injuries ,and, on this occasion, a month’s loan suits us and it suits the player perfectly.”
Montrose, 24, began his career with Wigan Athletic before joining Wycombe Wanderers in 2009. He made 50 appearances for the Chairboys before signing for Gillingham in 2011.
The 6ft central midfielder has made 18 appearances for Martin Allen's side this season, scoring twice. United fans may also remember the goal he scored against the U’s last season.
Montrose will not be involved today or in Tuesday's Johnstone’s Paint Trophy area semi-final against Southend United as he is cup-tied.
Wilder said: “Lewis has played over 100 games and is a very good player at this level.
“With Lee Cox and Jake Forster-Caskey returning to their parent clubs we wanted to add a midfielder to strengthen that department and we are fortunate to be able to bring in someone of Lewis’s quality. “Josh is someone we are willing to take a chance on and he deserves a chance to show what he can do over a longer period where he can get himself really fit.”