Matt Crooks hits post as Rotherham United beaten at home
The visitors were without at least nine of their usual starters at the New York Stadium but still came away with victory.
Rotherham struck the woodwork inside of seven minutes with Matt Crooks heading against the near post and both Chiedozie Ogbene and Ben Wiles unable to turn in the rebound.
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Hide AdThe Millers had had the better of the chances and the possession but they were stung by the opening goal at the other end on 42 minutes.
Rochdale built patiently through teenager Luke Matheson before he picked out fellow youngster Aaron Morley who fired an unstoppable shot into the top corner from the edge of the box.
Michael Smith was brought on at half-time as Rotherham went to a more attacking approach.
It was Crooks, again, who looked the most likely but his effort bounced just wide of the post.
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Hide AdRotherham then threatened down the right through Matt Olosunde and the full-back's cross picked out Clark Robertson, only for Robert Sanchez to keep hold of the defender's effort.
The visitors could have doubled their tally but Matt Done's shot whistled just wide and then Callum Camps tested Daniel Iversen from a long-range free-kick.
Rotherham: Iversen, Olosunde (Vassell, 73), Robertson, Ihiekwe, Mattock, Morris (Hastie, 60), Wiles, Lindsay (Smith, half time), Ogbene, Crooks, Ladapo.
Unused: Price, Thompson, Barlaser, Clarke.
Rochdale: Sanchez, Matheson, O'Connell, Norrington-Davies, Keohane, Camps, Ryan (Gillam, 66), Morley, Pyke (Baah, 76), Henderson (Tavares (90+2), Done.
Unused: Lynch, Bradley, Hopper, Brierley.
Referee: Christopher Sarginson.
Attendance: 8,313 (262 from Rochdale).