United now expect the 30-year-old striker to return in midweek in the Capital One Cup against Liverpool or next weekend against West Brom in the Barclays Premier League.
Moyes confirmed: "We thought Robin would be okay. We gave him every chance. It's nothing serious. He could be involved on Wednesday [against Liverpool] or next weekend [against West Brom]."
After the game, the United manager revealed more about the injury: "I don't think it's that bad, we thought he was going to make it. He just felt something in his groin.
"He did a bit of training in the last couple of days, but he just felt it when he did any sharp movements and he didn't want to aggravate it, so we took the decision [to not play him today]."