Healy has joined from Sunderland for an initial month and Rovers hope the 31-year-old can prove to be the ideal replacement for Billy Sharp, currently out with a torn hamstring.
Healy was handed a place in the starting line-up and the Northern Ireland international made an instant impact by striking a 58th-minute winner, two minutes after James Hayter had cancelled out Steve Morison's 11th-minute opener.
Rovers made two changes from last week with Wayne Thomas back from injury and Healy handed his debut up front.
Morison struck in the 11th minute when he latched onto a header by Kevin Lisbie and swept home a right-footed shot from six yards.
It was just reward for the visitors' early attacking vibrancy but Doncaster sought to find an immediate response.
A dangerous outswinging corner from James Coppinger caused panic in the visiting penalty area but David Forde punched the ball clear.
Millwall midfielder James Henry fired wide from 25 yards but two minutes later Rovers fashioned a fine move upfield.
It culminated in Healy firing goalwards from 20 yards but visiting goalkeeper David Forde saved comfortably.
In the 35th minute, Millwall could easily have gone further ahead when Morrison collected the ball just into the Rovers half and fed Liam Trotter running through to the left of goal.
His low shot from 18 yards was tipped around a post by Sullivan for a corner and then shortly before half-time Brian Stock tried his luck from distance but his shot went high and wide.
Millwall sought to press home their advantage after the restart when Dean Shiels fired goalwards from the edge of the penalty area and Forde made a fine parried save.
In the 50th minute, a teasing free-kick from Stock was greeted by a firm header from Shelton Martis, but his effort flew over the crossbar from 12 yards.
Five minutes later, Rovers were level when Morison fouled Shiels and Joseph Mills crossed from the right flank.
Hayter headed into the bottom right-hand corner from six yards out and two minutes later Healy struck with his first goal since February.
The ball fell the former Preston striker 12 yards from goal and he swept the ball home right-footed in clinical fashion.
Healy almost had his second moments later but fired just wide and John Marquis almost drew Millwall level but fired wide from 12 yards out as Doncaster held on.