United could be without winger Craig Conway for the quarter-final clash.
Scottish Cup winners United return to Fir Park eager to avenge last Saturday's 2-1 loss in the Scottish Premier League and secure a league cup semi-final spot.
Conway passed a late fitness test only to play just the first 35 minutes of last Saturday's match, having suffered a recurrence of a recent injury after suffering a bang behind his knee in training last Thursday.
And Tuesday's match may come too soon for Conway, who has been limited to seven appearances this season and scored twice in last season's Scottish Cup success over Ross County at Hampden Park.
"Conway is doubtful," said United boss Peter Houston.
"He got knocked accidentally just in the back of the knee and the bottom of the hamstring goes into the knee - we're almost positive it's that.
"He had his knee scanned and there's no swelling in the knee.
"If it had been anything to do with his knee you'd have got swelling."
Keith Lasley is an injury doubt for Motherwell.
He suffered a hamstring injury against the same team at the weekend, but boss Craig Brown hopes he will be fit for the cup clash.
Long-term casualty Esteban Casagolda remains on the sidelines.
Motherwell (from): Randolph, Reynolds, Craigan, Hammell, Jennings, Humphrey, McHugh, Lasley, Saunders, Hateley, Gow, Sutton, Murphy, Blackman, Fitzpatrick, Forbes, Page, Hollis.
Dundee Utd (from): Pernis, Dillon, Watson, Kenneth, Severin, Dixon, Douglas, Gomis, D Robertson, S Robertson, Buaben, Conway, Goodwillie, Daly, Cadamarteri, Shala, Russell, Hill, Dow, Banks.