The Ibrox boss was hoping to mark his first six months in charge of the Light Blues by leading them out against Celtic on December 2.
But Dons youngster Ferguson - son of former Ibrox hero Derek and nephew of ex-skipper Barry - wrecked those plans as headed home the winner with 11 minutes left.
Aberdeen have had a frustrating start to the campaign but they held firm to book Derek McInnes’ fourth cup final from five years as Pittodrie manager.
Gerrard decided to gamble on misfit frontman Umar Sadiq up front despite the on-loan Roma striker having previously registered just 26 minutes of action this term.
But with top scorer Alfredo Morelos suspended and Kyle Lafferty cup-tied, the 21-year-old got the nod for his first Gers start as he went up against Scotland defender Scott McKenna, back in the Aberdeen line-up after his two-game ban.
The Dons’ failure to sell out their ticket allocation left the 32,000-strong Ibrox faithful outnumbering the Red Army three to one.
Yet it was the Aberdeen section that were leaping out their seats just 33 seconds in as Joe Worrall’s shaky pass left Ovie Ejaria exposed.
Forced to bring down Lewis Ferguson, the on-loan Liverpool youngster was booked but there was no more punishment as Allan McGregor parried Niall McGinn’s free-kick for a corner.
Referee John Beaton flashed yellow again at Dons skipper Graeme Shinnie as he clattered into Daniel Candeias but that early flurry did the trick as it put the lid on the brewing feistiness.
Sadiq’s big chance came just five minutes in as James Tavernier picked him out with a free-kick but there was no dream start as he headed wide.
Aberdeen were creating openings too but McGinn’s composure failed him as he blasted over from Scott Wright’s cut-back.
McInnes’ team were working hard to shut down the areas of space Gerrard’s side normally thrive on, forcing the Light Blues to try their luck more the once from distance without much success.
Luck was nearly on Aberdeen’s side when a Gary Mackay-Steven cross almost dropped over McGregor’s head but the Ibrox keeper did well to tip over.
Beaton added eight minutes of injury time after Andy Considine suffered a nasty bang of heads with Lassana Coulibaly that saw him taken off on a stretcher.
That left time for a Rangers penalty appeal but there was relief for Shinnie as the officials spotted the toe he got to the ball as Ryan Kent went down.
The Ibrox side looked to force the issue after the break by upping the tempo.
Tavernier and Coulibaly came close but Aberdeen continued to stubbornly resist as the second period grew tense - and their bravery paid off with 11 minutes remaining.
McGinn’s whipped corner pinned the Rangers defence down under their own crossbar, leaving them powerless as Ferguson crashed home with his head.
Gerrard looked to respond as he threw on Glenn Middleton and Scott Arfield but the hapless Sadiq copped a yellow card as he took a dive at Joe Lewis’ feet.
Gers sent McGregor up for a stoppage-time corner but there was no dramatic rescue as the Dons clung on.