The effects of Covid-19 had left Darren Moore's team with a depleted team lacking elegant midfielder Ben Whiteman and a side which on paper, at least, was as half-and-half as the shirts of their opponents Blackburn Rovers.
Their defensive resources came in handy, marshalled by centre-back Tom Anderson, wearing the armband in Whiteman's absence. At home, and with a team as overflowing with midfielders as Doncaster's was short on them, Championship Blackburn unsurprisingly made most of the running, but it was the Yorkshireman who headed back over the Pennines with a 1-0 win.
With a back four composed entirely of centre-backs, a defensive midfield line of full-backs (if you can pin the versatile Reece James to a position any more) and four attacking players including debutant Elliot Simoes in front of them, Doncaster were well set up to counter-attack and they did it to perfection in the first half.
The Championship hosts, who in stark contrast to the other Rovers had a surfeit of midfielders, saw two thirds of the ball in the opening half but, not helped by a cabbage patch of a pitch, they struggled to do much with it. Stewart Downing pulled a shot well wide but beyond that, the early stages were all about passes going astray and crosses cut out by the consistently excellent Anderson's reading of the game.
Whiteman's quality will be missed if he does move on this month. Not for the first time, Doncaster go into a transfer window with their captain in demand from Championship clubs, this time led by Preston North End. The official line was that the elegant midfielder was missing through the effects of Covid-19 but with all the speculation around him, the timing was lousy.
Doncaster had not played since December 22 because of the virus, and were only able to name seven of the nine permitted substitutes.
His, though, is a team built to be comfortable on the ball and when Doncaster were able to get it down and play, they caused more trouble than their supposed superiors.
In the 12th minute former Middlesbrough goalkeeper Aynsley Pears, son of manager Tony Mowbray's old team-mate Stephen, dud well to come out and save with his feet from Tyreece John-Jules after the latter linked well with Taylor Richards, given the ball when makeshift midfielder Joe Wright won it back.
Five minutes later the pair linked up again, Wright's partner James starting the move this time. When Richards was found by John-Jules the Brighton and Hove Albion really ought to have hit the target, but squirmed his shot wide.
As the half-hour mark approached, Blackburn looked as if they might get it together but Ben Brereton curled wide, Bradley Dack thumped a free-kick beyond the post and Balcombe got down well to cut out John Buckley's cross. Anderson blocked a Downing shot and Darragh Lenihan was unable to stretch to Tom Trybull's very dangerous cross.
Blackburn failed to hit the target with any of their nine first-half attempts. The visitors had only three but made the last of them count.
James fed Richards on the halfway line and from there he was able to carry the ball deep into Blackburn territory without a challenge and place a shot Pears should not have let through him and into the net.
Blackburn continued to pile on the pressure and the substitutes in the second half but Doncaster stood firm.
Balcombe's footwork was a big reason why Moore loaned him from Brentford and he generally justified that faith, but over-confidence gave the hosts a glimmer of hope at the start of the second half, allowing Dack to close him down and relieved to see his block drop just the right side of his goal.
It would not be the last moment of good fortune for the visitors, but underdogs need it in the FA Cup and they certainly earned it. It was the first and last Balcombe mistake.
When Downing took a corner his deflected shot had won, Anderson did brilliantly to head the ball onto his own crossbar and keep Balcombe's sheet clean.
Mowbray brought on teenage Liverpool loanee Harvey Elliott. He and Dack both shot wide before both being denied by Balcombe.
The 72nd-minute save from Elliott was impressive, leaping up to deny Dack from point-blank range was outstanding.
Blackburn threw on 17-goal Adam Armstrong for the final ten minutes to try to find the equaliser but inevitably Anderson through himself in the way of a goal-bound after 84 minutes. It was that kind of performance from the no-nonsense centre-back.
Even so, Blackburn ought to have taken the tie to extra-time, John Buckley getting to faint a touch on a brilliant curling ball from substitute Joe Rothwell.
Mowbray will be wondering how his side is out at the third-round stage for the fourth season running but for their effort, their discipline and their heroic defending, Doncaster deserved it.
Blackburn Rovers: Pears; Buckley, Johnson, Lenihan, Bell (Douglas 46); Trybull (Armstrong 64), Travis (Elliott 58); Dolan, Dack (Davenport 73), Downing (Rothwell 58); Brereton.
Unused substitutes: Kaminski, Gallagher, Grayson, Pike.
Doncaster Rovers: Balcombe; Halliday, Anderson, Butler, John; Wright, James; Okenabirhie (Amos 80), Richards (Lokilo 67), Simoes; John-Jules.
Unused substitutes: Jones, Williams, Coppinger, Blythe, Ravenhill.
Referee: S Barrott (Hertfordshire).
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