An intense rehabilitation programme followed, enabling him to return to the pitch in February.
But just as he was pulling on the claret and amber again, Covid-19 forced football to shut down.
Working hard on his fitness during lockdown, the 21-year-old hit the ground running when the Premiership season kicked off last month, and standout displays during a sticky start for Motherwell encouraged Celtic to come back in and finally get their man.
Last week, Turnbull signed a life-changing four-year contract with Celtic and, in the process, his club record transfer fee of £3m earned Motherwell a huge cash injection in uncertain times.
One man who has watched Turnbull’s trials and tribulations along the way is Coltness High School head teacher, John McGilp.
McGilp coached a teenage Turnbull when he was in S4, playing two years above his age level, and he insists the mental fortitude Turnbull has shown over the last year is no surprise to him, given what he had seen at close-quarters.
In May 2015, Turnbull’s path to Parkhead began when Motherwell boss Ian Baraclough signed him on his first professional contract.
And McGilp has witnessed the Coltness kid grow in stature and his talent shine.
He said: “I was fortunate enough to have David in Craig Barnstaple and I’s senior football team when he was in S4.
“He played against St Margaret’s in Airdire, who at that time were the reigning Scottish Schools’ champions.
“At that time he looked so out of sorts, two years young and very slight. But what he lost in stature at that time he made up for in guile, touch and a fantastic footballing brain.
“You could see that everyone on the pitch, being so much physically stronger, had a couple of yards on David, but his intelligence on the ball and positional sense made sure that he not only made up these yards, he kept a step ahead of opponents.
“What really struck me about him was how he played constantly with his head up; not only looking for the ball but watching team-mates and opponents alike, knowing where they were at all times.
“His physique as an S4 was way out of line with the boys two years above and when he left the school to go full time, that was my only concern – his ability to transition to the level he has.
“His quest to do this, however, was relentless.
“Every time he was up at the school dropping something off for us, he’d just been at Wishaw Sports Centre for an individual workout following his Motherwell training.
“As a result he’s turned into a very physically strong player, which probably stood him in good stead last year for getting over his operation.
“Last year my heart sunk for him, given the predicament he was in.
“Things were sailing along brilliantly and then he hits an iceberg at the 11th hour.
“The mental strength he has gained through this time will stand him in good stead for what is to come in terms of being involved with one of the Old Firm clubs.”
As a child Turnbull stood out as young as nine-years-old, playing for local side Newmains Hammers when he was pupil at Lammermoor Primary.
In 2009, Motherwell’s youth academy beat Celtic and Rangers to the Wishaw wonderkid’s signature.
Hammers coach Allan McNeil even said at the time: “I’m sure Motherwell will have a tough time in the future keeping the big guns of Scottish football at bay.
“I know this because over the past few weeks my phone has been ringing constantly from SPL teams enquiring about David.”
While developing at Fir Park, Turnbull represented North Lanarkshire at the International Children’s Games in 2014, playing alongside Motherwell team-mate Jake Hastie.
But he came to the fore in 2016, helping Motherwell to a Youth Cup win, and made his senior debut two years later in a 2-0 win over Dundee in a Scottish Cup fifth round tie, aged 18.
McGilp added: “Whilst he was still at school I had the pleasure, and much pride, of watching him represent The Scottish Schools FA up at McDermott Park against Wales.
“I then managed to go to Hampden to cheer on the Claret and Amber as he starred in a Motherwell U20 side that won the BP Youth Cup, soundly beating Hearts 5-2 on the night.