Its value for Cagliari was a 2-2 draw at San Siro, the diminutive striker slipping the ball into an empty net after leaving Internazionale defenders Andrea Ranocchia and Walter Samuel and goalkeeper Samir Handanovic lying helplessly on the turf.
It was the goal that knocked Inter's title challenge off course, breathed new life into what had looked like a nightmare campaign for Cagliari, and made Sau's name. But what few realise was that it was also a goal made in Gourock.
All of this took place last month in Milan, and since then the hype surrounding Sau has been building. The goal was his second of the game – the first a superb half-volley that flew into Handanovic's top corner – and has ensured that the 25-year-old is the subject of much fuss in Sardinia. Yet it was on the banks of the Clyde, in the unlikely environs of Gourock Youth Athletic Club, that Sau underwent an important part of his football education.
"Marco played for us when he was about 13 or 14," explained Robert Ahlfeld, the former YAC coach. "He was on holiday visiting his uncle Gianni, and Gianni was looking for somewhere for him to play while he was over. He'd been at Livingston, and for whatever reason, that hadn't worked out."
It didn't take long for Ahlfeld to realise he had a gem on his hands. "Instantly I knew he was talented, a bit special. But he also had something else – he adapted really quickly," he said. "He had this natural affinity with a lot of the kids, even though his English wasn't great. His technique, touch and vision were absolutely out of this world at that age. We used to play in some really tight games and he would decide them, it was exceptional some of the goals he could score.
"He actually stayed longer than planned because he liked the kids and enjoyed himself, until time dictated that he needed to go back home. Actually, I think that was less to do with his football connections and more to do with his mother missing him. Basically it was an extended holiday and he got homesick."
Just a few years after returning from Scotland the teenager was signed by Cagliari and, like so many youngsters in Serie A, Sau was forced to prove himself during various loan spells to the Italian lower divisions. His big break came at third division side Foggia, where he scored 20 goals under the guidance of Zdenek Zeman, the eccentric, football-obsessed Czech who is now coaching Roma.
Sau then proved himself in the second tier by scoring 21 times for Juve Stabia, before being inducted into the Cagliari first team last summer, five years after signing for the Rossoblu. He started the season on the bench but the installation of Ivo Pulga and Diego Lopez as co-coaches following the sacking of Massimo Ficcadenti brought a regular starting berth.
Goals against Palermo and Siena followed, before stealing the show at San Siro. He turned in another impressive display against Napoli the following week and is now being linked with a move to the southern club as a result.
With his ability, short stature and tendency to leave socks rolled down around his ankles, Sau is reminiscent of another son of Sardinia, Gianfranco Zola, who spent the last two seasons of his career with Cagliari after leaving Chelsea. The local press have been unable to contain their excitement over the similarities either, not that they have tried, La Nuova Sardegna describing the forward as: "169cm of pure class".
Unaware of Sau's recent exploits, Ahlfeld was not given of such hyperbole but neither is he surprised to hear of the striker's rise to prominence in Italy. "I always wondered how he'd got on," he said. "I'm delighted that he's made such progress. To have that ability . . . it struck me that Scottish clubs weren't on to it quicker.
"I like to think I helped him a wee bit at that age. He was exceptionally gifted, he was a nice boy, and he worked his socks off in training. He is a natural-born footballer."
Sau dedicated his goals against Inter to his "girlfriend Jade and all those who love me". Perhaps in future – whether scoring for Cagliari, Napoli or even the Italian national team – he will afford similar recognition to his old friends at Gourock YAC.