The Fir Park side resisted a Ross County comeback from 2-0 down, with Liam Boyce striking a retaliatory blow, but trudged off to find St Johnstone had claimed a crucial win over Champions Celtic.
In a hard-fought encounter, Well dominated the first half but had to dig deep to thwart County’s lingering hopes of finishing fifth. But with Pearson claiming the killer second in stoppage time, McGhee’s pre-match words rang true.
With Motherwell still to face Celtic in their last game, this wasn’t quite win-or-bust for County’s chances of fifth, but they couldn’t afford to lose.
Well certainly looked equipped for the task – and made a brilliant start.
After winning a brace of early corners, they took a fourth-minute lead, slicing through home defences with alarming ease. Scott McDonald found Louis Moult on the right and the home rearguard seemed to part obligingly as the Englishman cut the ball back to leave Pearson with the simplest of finishes.
The second came inside 16 minutes with Stevie Hammell’s corner eventually breaking conveniently to Lasley, who produced something of a personal collector’s item with a 25-yard strike low into Scott Fox’s right-hand corner.
Five minutes into the second half, on-loan David Goodwillie miscontrolled in front of goal from Michael Gardyne’s decent cross and it was a similar story not long after when Martin Woods sent in another dangerous ball. Goodwillie kept grafting and drew a superb close-range parry from keeper Craig Samson approaching the hour mark.
Alex Schalk was guilty of a howler after 67 minutes, shooting wide from six yards from a Graham knockdown, but County’s luck turned with substitute Boyce lifting the home crowd when he nodded in from close-range after 70 minutes after Gardyne drove at goal from inside the right corner of the box and Samson parried.
The Dingwall team had the bit between their teeth, surging forward, and Brian Graham saw a fine, dipping header tipped brilliantly for a corner by Samson, but Pearson killed it in stoppage time with a headed third.
ROSS COUNTY: Fox; Robertson, Quinn, Irvine, Foster; Franks (Boyce 56), Murdoch (De Vita 36), M Woods, Gardyne; Schalk, Goodwillie (Graham 60). Subs not used: G Woods, McShane, Boyd, Dingwall.
MOTHERWELL: Samson; Cadden, Hall, McManus, Hammell (Chalmers 72); Johnson, Lasley, Gomis, Pearson; McDonald, Moult. Subs not used: Ripley, Leitch, Campbell, Thomas, Ainsworth, Laing.