United suffered a first defeat to Leeds at Old Trafford in 44 years.
“Over the last day or so, Kobbie just had an issue in training,” Carrick told Sky Sports, when asked about Mainoo’s absence before kickoff.
“So it seems small but big enough to keep him out tonight. It wasn't worth the risk, to be honest, so we're hoping it's pretty small and it won't be too long.
"Manu (Ugarte) comes in tonight. He came on and did well for us in the last game at Bournemouth and went away and played well for Uruguay so he's well up to speed.
"He's been great in training and done great all the way through really and part of what we've done, even though it's not obvious because Kobbie and Case (miro) have played a lot.
"So yeah, comes in tonight, big opportunity for him to show himself and we deal with Kobbie being out."