The name of Ross Turnbull raised a few eyes when he was named to start the game with manager Tony Mowbray confirming why by saying: "Ross has trained with us for a couple of weeks. He wanted to keep fit after he left Chelsea in the summer.
"I thought I'd just take a look at him tonight in a match. Jayson Leutwiler and Connor Ripley are playing in the U21s against Darlington."
Turnbull kept goal behind a back four, three of whom boasted just one first team start between them.
It took a little while to settle, the Millers winning three corners in quick succession, but Boro eventually found a decent rhythm.
The lead came on 15 minutes after Andy Halliday was brought down a couple of yards out from the right-hand edge of the penalty area. Captain Richard Smallwood swung over a cracking delivery and Ben Gibson powered home a header from seven yards.
The lead lasted two minutes and a further two on from that Boro were behind.
A foul inside the box by Faris Haroun on Claude Davis was spotted by referee Howard Webb and Michael O'Connor rammed home the penalty.
A mistake from Christian Burgess laid the second on a plate for Alex Revell, who took control close to the edge of the penalty area and drilled low past Turnbull.
A period of relative calm descended following three goals in four minutes, before another little flurry of activity saw United forced into a substitution after the lively David Worrall limped off and Gibson head a good chance wide of the mark having taken up a good position to meet another fine Smallwood delivery, this time from a corner.
A chance at either end could easily had added to the score as Turnbull saved well from Kieran Agard and Halliday saw a low shot turned round for a corner.
With five minutes of the first half remaining Cameron Park set up Faris Haroun, who was left with a clear run into the penalty area where he rounded the goalkeeper and rammed home at an angle.
Half-time: Rotherham United 2 Middlesbrough 2
As planned, Jason Steele replaced Turnbull at the start of the second half. It was one of three changes, Rhys Williams for Gibson and Lukas Jutkiewicz for Haroun being the others.
An open start to proceedings in the second half saw Rotherham win a couple of corners in quick succession and Boro go close to regaining their briefly-held lead.
Park peeled away down the left and was picked out with a ball that allowed him time to advance into the penalty area only to see Adam Collin produce a fine save.
Steele did well to deny a determined Agar before Boro edged into to the lead when Lukas Jutkiewicz timed a run well down the right, moved into the penalty area, rounded the goalkeeper and slotted home.
Agard had proved a real handful following his earlier than anticipated introduction and a low shot deflected off Adam Reach for a corner.
Further changes saw Bryn Morris and Ryan Brobbel introduced as Matty Dolan and Halliday made way and for a fleeting moment it looked as though Park may follow after he was clattered by Claude Davis.
Determination and opportunism allowed Rotherham to put in a good finish, during which Revell forced Steele into an excellent save from a close-range header and Agard work the Boro goalkeeper with a low shot from the edge of the box.
It was a game in which Boro were asked lots of questions. Andre Bennett can be well pleased with his first 90 minutes in a first team shirt, while Richard Smallwood was Mr Dependable. As a team, they all knew they had been in a game.
Scorers: Gibson 15, O'Connor (pen) 17, Revell 19, Haroun 40, Jutkiewicz 61.
Rotherham: Collin; Brindley (Bradley 66), Skarz, Arnason (Tidser 57), Davis (Capt), Worrall (Agard 34), Revell (Hylton 81), O'Connor (Noble 57), Milsom (Rose 66), Pringle, Morgan. Unused sub: Thompson (gk);
Boro: Turnbull (Steele 46); Bennett, Gibson (R Williams 46), Burgess, Halliday (Brobbel 63), Haroun (Jutkiewicz 46), Smallwood (Capt), Dolan (Morris 63), Reach, Ledesma, Park. Unused subs: Leadbitter, Emnes, Carayol, Whitehead, Parnaby, Varga.
Referee: Howard Webb
Attendance: 2,143 (271 Boro)
Conditions: Heavy rain and thunder leading up to the start, then fine, warm and clear.