16-year-old Luke Douglas makes his first start for the senior team as Craig McLeish makes five changes to his starting lineup.
The interim Saints boss rotates his squad ahead of Thursday's first leg Premiership Play-Off against Partick Thistle.
Jayden Richardson, Roland Idowu, Scott Tanser and Jake Young come in from the start alongside Douglas. Tuesday night's goal-scorers Richard King and Killian Phillips are on the bench as are Liam Donnelly, Jacob Devaney and Mikael Mandron.
St Mirren: Sinclair, Fraser, Gogic, Freckleton, Richardson, Idowu, Campbell, O'Hara (c), Tanser, Douglas, Young
Subs: Tamosevicius, Donnelly, King, Devaney, McMillan, Phillips, Falconer, Calvin, Mandron
Dundee Utd: Richards, Strain, Keresztes, Cleall-Harding, Graham (c), Gilligan, Sevelj, Camara, Sibbald, Sapsford, Watters
Subs: Adams, Esselink, Eskesen, Naamo, Stirton, Constable, Mudie, Russell
Referee: Steven McLean
VAR: Duncan Williams
