Matt Crooks’s 11th minute goal has given Rotherham a 1-0 lead over the Blues at Portman Road at half-time.
Town boss Paul Lambert made five changes from the team which lost 2-0 at Accrington in Sunday.
James Wilson returned to the centre of Blues’ defence, having been suspended at the weekend, with the Blues starting with a four-man backline. Luke Woolfenden moved to the bench and Toto Nsiala dropped out of the 18.
Kane Vincent-Young was at right-back having missed the Accrington game with his groin injury with Gwion Edwards was on the bench.
In midfield, Emyr Huws replaced Andre Dozzell, who also missed out on a place in the squad, alongside Cole Skuse, while Danny Rowe also came into the team behind the front two with Alan Judge among the subs.
Up front, Will Keane started alongside Kayden Jackson with James Norwood and Flynn Downes both back on the bench after groin surgery and a hip knock respectively.
Former Blues keeper Lewis Price was on the bench for the Millers alongside one-time Town trialist Chiedozie Ogbene.
Rain fell steadily and persistently as the game got under way. Within two minutes a pacy Jackson run down the left won a Blues corner from which they went close to taking an early lead.
Rowe claimed he was tugged as he sought to get on to the loose ball after Rotherham had failed to clear but referee John Busby was unimpressed.
The ball eventually reached Huws at the back of the box from where he struck a shot which Millers keeper Daniel Iversen pushed into the air. Keane was first to the ball but headed straight at the keeper.
The wet conditions were having an impact on both sides in the early stages with Rotherham struggling to clear their lines on a number of occasions but with the Blues unable to profit.
The Millers hadn’t seriously threatened but in the 11th minute they took the lead. Skuse felt he was fouled as he held off a Rotherham player on the edge of his own box but referee Busby felt otherwise and Ben Wiles sent the ball back in from the left and it fell for Crooks, who struck a powerful low effort which skimmed across the wet pitch and through keeper Tomas Holy’s legs.
Rotherham should have made it 2-0 two minutes later when Joe Mattock found Matthew Olosunde breaking into the area at the far post with a cross from the left but the US international shot over when he should have hit the target at the very least.
The visitors continued to look the more threatening side and on 25 Trevor Clarke whipped over a freekick from the right which Crooks only just failed to get his head on.
Three minutes later Clarke was booked for a very late challenge on Vincent-Young, hardly the first Rotherham challenge which had warranted a yellow card.
On 30 Mattock was spoken to by the referee for time-wasting with the Millers taking an age with every restart having gone in front. In the 42nd minute Wiles was booked for kicking the ball away as Town prepared to take their 11th freekick of a very stop-start half.
Just before the whistle, Mattock crossed from the left and Michael Smith headed over at the far post.
It had been a scruffy, disjointed half from the Blues who had struggled to pass the ball with any fluidity - the damp conditions playing a part - and aside from the early double chance had rarely looked like scoring.
Rotherham took their chance when it came and might well have doubled their lead soon afterwards. From there, they looked to disrupt the flow of the game and frustrate the Blues but when they had the ball also managed to create one or two further openings.
Town need to be a lot better after the break and, as at Accrington on Sunday, a couple of changes ahead of the second half wouldn’t be a surprise.
Town: Holy, Chambers (c), Skuse, Wilson, Vincent-Young, Nolan, Huws, Rowe, Garbutt, Jackson, Keane. Subs: Norris, Edwards, Norwood, Judge, Downes, Woolfenden, Georgiou.
Rotherham: Iversen, Jones, Wood (c), Ihiekwe, Mattock, Olosunde, Wiles, Lindsay, Clarke, Crooks, Smith. Subs: Price, Barlaser, Lamy, Ogbene, Hastie, Morris, Ladapo. Referee: John Busby (Oxfordshire).