But it will have been the sight of Ashley Young back from suspension and Carew completing 90 minutes, that will have warmed the Villa manager the most on a bitterly cold night in Birmingham.
With Arsenal just five points behind in the league, a date with CSKA Moscow in the Uefa Cup looming and the fact that several players will be lost to international duty next week, the FA Cup has been downsized on the Villa priority list.
'Prioritising: I'm beginning to hate that word,' O'Neill said after the match. 'I love the FA Cup, and I am delighted to be in the fifth round, but if the Premier League is the holy grail then we have to be chasing that.'
The Villa manager had promised changes and he left Brad Friedel, Gabby Agbonlahor, Gareth Barry and James Milner on the sidelines. Emile Heskey, meanwhile, was unavailable, having not been signed in time for the original tie two weeks ago.
Their absence could have proved costly for a side who have often struggled to find the net at home this season, but O'Neill was once again call on Young, back from his three-game suspension. The winger made the most of his enforced absence by flying to Dubai for a few days in the sun, and the rest seems to have done him good.
Plying his trade with a new pair of charcoal-coloured boots, Young no longer goes about his business in a blur of flashing white feet. Otherwise he was back, bobbing and weaving on the wing as usual. It must have been comforting for O'Neill in the fourth minute as Young cut in from the left and arrowed a right-footed shot just over the bar.
It was no surprise that it was the winger who was behind the first goal of the night. Young picked out Craig Gardener with an astute ball into the box in the 15th minute and Doncaster goalkeeper Neil Sullivan, a brick wall for his side a fortnight ago, first blocked but then spilt the subsequent shot, straight to Steve Sidwell to tap in from point-blank range.
It was a bitter blow for the visitors and their plight was made worse four minutes later when Carew brought a loose ball under control with his knee outside the area and then stroked a volley goalwards. Jolted off its original course after clipping Sam Hird mid-flight, the ball sailed over a helpless Sullivan.
Yet Doncaster refused to be dismissed, and came forward time and again, going close when James Coppinger, a sprightly character on the wing, sent a dangerous cross into the Villa box, which was headed clear by a quick-thinking Nicky Shorey.
Coppinger was soon back in the mix though, angling for more involvement. Taking a leaf out of Young's book, he switched flanks and almost immediately took advantage of a scuffed clearance from Delfouneso on the edge of Villa's area, and crossed to Jason Price for the striker to send the ball beyond Brad Guzan into the bottom right corner. It was Price's first goal of the season and it couldn't have come at a better time, right on half-time.
Doncaster emerged from the break looking to take the initiative early on, and came close with a shot from Brian Stock 10 minutes in. But it was Villa who came up with the goods, Shorey popping up with a swinging cross from the left to Delfouneso, who headed the ball past Sullivan in the 61st minute.
Doncaster heads dropped. Fighting from behind, they never looked likely to offer a comeback, leaving Villa to coast into their first FA Cup fifth round on O'Neill's watch.
Aston Villa (4-4-2): Guzan; Luke Young,Cuellar, Davies, Shorey; Gardner, Salifou, Sidwell, Ashley Young (Osbourne 73); Delfouneso, Carew.Subs: Friedel, Barry, Milner, Agbonlahor, Knight, Lowry.Booked: SidwellGoals: Sidwell 15; Carew 19; Delfouneso 61.Doncaster Rovers (4-5-1): Sullivan; O'Connor, Mills, Hird, Chambers; Spicer (Guy, 75), Coppinger (Byfield, 76), Stock, Wellens, Woods; Price (Heffernan, 75).Subs: Roberts, Van Nieuwstadt, Taylor, Lockwood.Booked: HirdGoals: Price 45.Referee: L Mason (Lancashire)