The 20-year-old defender, signed on loan from Tottenham, could hardly have written the script better himself as he scored a brilliant goal nine minutes into his debut against Sheffield United, the club he supported as a boy and where he began his career.
Marc Albrighton made it 2-0 before the break and Villa were able to withstand a second-half fightback by the Blades, for whom Jamie Ward scored from the spot, before Stiliyan Petrov - after Ashley Young's dismissal - secured their place in the hat for the fourth round of the FA Cup.
Houllier, who had included Robert Pires in his starting line-up despite his fellow Frenchman's critical comments this week, would have been encouraged as Villa made a blistering start at Bramall Lane.
Micky Adams' side looked intimidated and their feeble start was summed up perfectly when Walker strode through to give the visitors the lead.
The right-back slipped away from one challenge just inside the United half and advanced into space before side-stepping Rob Kozluk and slotting home past Steve Simonsen to set his side on their way to a comfortable win.