The 20-year-old from Bellshill tore through to grab the Steelmen’s third of a dramatic evening in Dingwall.
On a night of ragged, tireless football, County, the 2016 winners, foundered despite Charles Dunne’s sending off and after twice clawing their way back into the match.
Ross Draper’s six-figure flit across the Kessock Bridge to join Ross County, announced just a couple of hours before kick-off, dominated pre-match chatter among the home support.
The Caley Thistle stalwart of five years joined a sizeable list of players to have served on both sides of the Highland divide, but the deal was infused with greater significance than most. The Dingwall club, now a league above Inverness for the very first time, have for many years been accustomed to having their best players pinched by the neighbours.
The totemic capture of Scottish Cup-winner Draper, who wore the Inverness captain’s armband as recently as Saturday, underscored the power-shift now at play between the clubs.
County had failed to capture the 28-year-old Englishman in three previous five-figure bids but Draper’s transfer request forced Caley Thistle’s hand.
Draper passed a medical before the game to finalise the switch but was cup-tied in any case.
Motherwell settled first. Young Chris Cadden, lively and in the thick of it from the off, sent in a decent cross from the right but Craig Tanner’s header spun wide. As the Fir Park side went on the offensive, County’s Marcus Fraser also had to make a strong saving tackle to prevent Tanner closing in on goal.
County lost midfielder Tim Chow to a head injury after clashing accidentally with team-mate Andrew Davies, with ex-Motherwell man Jim O’Brien his replacement.
Carl McHugh’s strike from distance rose just over the bar as County struggled to get a firm foothold.
It was scrappy for lengthy spells and the breakthrough did not come until five minutes into the second half. A surging, twisting run from Cadden on the right ended with a cross for the head of Louis Moult.
The big striker knocked his header sideways and there was Gael Bigirimana, pictured, racing in to beat Scott Fox from eight yards.
Schalk, after twice missing chances, didn’t shirk his third chance of the evening when he drew County level after 65 minutes.
The Dutchman raced on to a tremendous long through pass from O’Brien and expertly lofted a 12-yard shot past outcoming Carson.
When Ryan Dow rashly felled Cadden just outside the County box, Moult teed up the free-kick but stabbed it wildly over the bar.
Gardyne spared County with ten minutes remaining when he headed a Fisher attempt off the line. But Cadden took just three minutes of extra-time to restore the Motherwell lead.
From Alex Fisher’s cross, Dunne’s header smacked the bar but dropped near the right hand post for Cadden to force home. County dragged themselves back into it at 2-2 when Dunn was red-carded for fouling Craig Curran right on the goal-line and the Liverpudlian netted the penalty himself.
Even a man down, Motherwell weren’t beaten – and MacLean raced through onto a Cedric Kipre through pass to beat Fox for the clincher.