The forward, on loan at Belgian club Lierse until the end of the season, had been accused of being unfit and lacking in workrate by new coach Stanley Menzo and Watt pulled his shirt off to display a message: his bare torso.
The Scotland Under-21 player had intended to show off his physique to prove that he was in good physical condition, although it came at the price of a yellow card. Watt later insisted he was prepared to accept a caution to get his irritation off his chest - as well as his jersey - while he was likely spurred on by the realisation he has now scored two goals in as many games for Lierse.
The public dressing down from Menzo had upset Neil Lennon as well, even if the Celtic manager could identify with the manager's desire to see a fitter Watt in action. The striker, though, is convinced that he is in top shape.
"I knew that I would receive a yellow card for taking off my shirt," he said. "But I wanted to show everyone that I am in prime condition and not fat. I am not overweight, that is just a myth which has somehow spread. Lierse have brought me here to score goals and I am pleased to have done that so early into my time in Belgian football."
That has allowed Watt to build his reputation in Belgium, beyond his goal for Celtic against Barcelona in the Champions League group stages last season. His parent club will meet the Spanish champions again at the same stage of the competition this season, with Barca visiting Celtic Park on Tuesday.
They will arrive without defenders Javier Mascherano and Jordi Alba since the pair have both picked up injuries, although the Catalan's rearguard is expected to be bolstered by the return of captain Carles Puyol. The Spaniard has not played for five months as he recuperated from an operation on a cyst in his knee, and club coaches are monitoring his fitness ahead of a first-team return against Celtic.
"I watched [Puyol] for a while and spoke to him," said Andres Iniesta, the Barca midfielder. "He said that everything is fine and that he is desperate to play an official match. He deserves it because of all the effort he has put in to being fit and it will be a great piece of news if he can play again. However, that depends on his fitness level and how he feels."
Iniesta has added to the good news at Camp Nou by insisting that he will sign a new contract, despite interest from both Manchester United and Manchester City. "My wish is to end my career at Barcelona. We are going to reach an agreement," added the midfielder.