The 79th minute winner allowed the player who was recently sidelined for eight months with glandular fever to get that particular monkey off his back but also gave his side three points and left Livingston pointless and at the foot of the Premiership table.
For the visiting side the celebrations were real as delighted team-mates mobbed the gutsy full-back, who had been deployed in his ancillary midfield role, but so too were the concerns in the home dug-out.
It was the kind of defensive display that forced manager David Martindale to revise his earlier opinion that any more transfer business would be a luxury rather than a necessity.
He will now endeavour to bring in some reinforcements as he tries to find the key to unlock the form his men showed last term and undoubtedly still possess.
It was a slow start last season as well. Livi went five games without a win at the start of that campaign but they mustered a couple of points, against the Fir Park side and Rangers, to help preserve some optimism.
Three games into the new term, with Hibs, Dundee and Celtic waiting for them in the next three head to heads, improvements are needed.
Martindale still firmly believes he can fix things and history suggests he is probably right. Last year they regrouped and turned themselves into challengers for a European place, while also progressing to the League Cup final.
And, there were periods in the game when they brushed forward with the kind of fast-paced, sweeping intent that can catch opponents off-guard. They just couldn’t capitalise on those moments often enough, with Nicky Devlin the worst offender with a second-half effort.
Livi opened the scoring in the 38th minute, after an earlier goal from Ayo Obileye was chalked off for a foul in the build up.
But there was no denying Alan Forrest, who clinically finished a pass inside from Bruce Anderson, low into the bottom corner.
It is a turn of events that will have stung Motherwell, who after a draw against St Johnstone and defeat to Hibs, had been seeking their first league win.
If the home side had the momentum at that stage, Motherwell, who had enjoyed sustained periods of pressure without the rewards, snatched it back three minutes after the interval when Juhani Ojala nodded on a ball and Watt headed it past Max Stryjek.
That Devlin miss was to prove pivotal as the marking at the other end ultimately handed Motherwell the winner.
They had been frustrated when a chance to shoot into a gaping goal was denied them as the ref halted play to deal with the injured Sean Kelly, who had been cleaned out by his own keeper.
While Motherwell manager Graham Alexander understood the need to consider welfare, he was infuriated when the official restarted play with a free-kick to the home side denying his team the advantage they had clearly enjoyed when the collision occurred.
But with 11 minutes remaining the guests did get their victory as Grimshaw followed up a blocked Watt effort and netted from close range to break his duck and Livi hearts.