The Millers acted late in the window as they signed Georgie Kelly (Bohemians, free) and Jordi Osei-Tutu (Arsenal, loan) in the final days of trading.
United hope the fresh additions will help them maintain their place in the automatic promotion places. Paul Warne's side currently sit top of the table, three points ahead of Wigan, although the Millers have played two more games.
Warne's men do have a five-point cushion on third-placed Sunderland and have a played a game fewer than all the sides placed between third and sixth.
Curtis Tilt (Wigan Athletic, undisclosed) and Kieran Sadlier (Bolton Wanderers, undisclosed) both left the club permanently during January while Jake Hull joined Hartelpool United on loan.
Further down the table, Sheffield Wednesday are hoping some January additions will bolster their chances of promotion as they aim to climb back into the top six.
Darren Moore strengthened his backline with defenders Jordan Storey (Preston North End, loan) and Harlee Dean (Birmingham City, loan) joining until the end of the season.
Tyreece John-Jules has arrived at Hillsborough on loan from Arsenal to give the Owls another option up front. Wednesday have won their last two games without conceding a goal to leave them four points behind the top six with a game in hand.
Below is how the bookmakers rate the top 12's promotion and play-off hopes after the closure of the January window.