Oxford boss Chris Wilder conceded his side's failure to deal with the threat of Rotherham marksman Adam Le Fondre was at the root of their 2-1 defeat at the Don Valley Stadium.
Le Fondre struck his 15th and 16th goals of a prolific campaign before Simon Clist reduced the arrears before half-time.
"Le Fondre is sharp in the box, but we knew that before and he's punished us," said Wilder.
"We started quite brightly and were on the front foot but then found ourselves two down pretty quickly.
"But we got ourselves back into the game and in the second half we tried to force the issue.
"We were in their half and the final third numerous times and with a bit more quality, a bit more decent delivery, then we would possibly have got something out of the game."