Romero, who played most of United's game en-route to Europa League glory last season, featured in 18 games in all competitions in 2016/17. He has had to wait until now for his opportunity, though, with David de Gea restored to European action for this season.
All have been automatic selections so far this season and will benefit from the rest against Burton.
"I gave two days off only to four players. Apart from that, everybody is selected. Do you want to know which four? De Gea, Valencia, Matic and Bailly," Mourinho said as he addressed the media ahead of the clash with the Championship club (ManUtd.com).
"Of course, we're not playing David or Bailly but, if we go to the next round, probably these guys will play and I will give a rest to others. Our squad is good."
Beyond long-term absentees Zlatan Ibrahimovic and Marcos Rojo (both knee), only Paul Pogba (hamstring) and Axel Tuanzebe (back) will miss out through injury.
De Gea's rest and Romero's promotion will see Joel Pereira take a place on the bench. The Swiss-born Portugal U21 international, who has been at United since 2012, is highly thought of by Mourinho and played twice for the first-team last season.
He was also involved in pre-season games during the summer, equally sharing playing time with De Gea and Romero in the first few tour fixtures.