West Brom, who face Newcastle in the fifth round of the FA Cup, extended their Sky Bet Championship lead to four points after an impressive 2-0 win at Millwall.
On-loan Benfica midfielder Filip Krovinovic opened the scoring by firing in from 20 yards just before half-time, while Dara O'Shea added a second late on as they handled the tough conditions of Storm Ciara.
After a seven-game winless run, West Brom have won two straight and are now comfortably clear at the summit again.
Having struggled against Rochdale and Oxford, United have a tough fixture if they want to progress to Wembley.
Steve Bruce says he will field his strongest team, altough I'm sure he will make a change in goalkeeper.
Full Emirates FA Cup fifth-round draw:
1 Sheffield Wednesday vs Manchester City
2 Reading vs Sheffield United
3 Chelsea vs Liverpool
4 West Bromwich Albion vs Newcastle United
5 Leicester City vs Birmingham City
6 Derby County vs Manchester United
7 Tottenham Hostpur vs Norwich City
8 Portsmouth vs Arsenal