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According to stats supremo Kevin Millar (Top of the Moon), no Glasgow Warriors player has achieved that feat in the past twenty years.
Handed a starting spot by Franco Smith, Watson wasted no time making his mark. He crossed for a try just a minute into the second half, capping off a memorable evening for the winger.
His debut could have come much earlier in the season, but a serious hamstring injury - sustained while playing for Emerging Scotland against a Tonga XV at Scotstoun - kept him out for more than six months. In the end, though, the wait only made the moment even sweeter.
"I was buzzing - it was class," said the former Scotland U20 man.
"I got injured playing against Tonga the week before the national team faced them. It was an international side, and when you get the chance to play at that level, you never turn it down.
"Injuries happen, but I feel like I've come back better, stronger, fitter, probably in the best shape I've ever been in. There's a positive side to it.
"I used it as an opportunity to improve other parts of my game, especially in the gym.
"And I just hoped the rugby would come back once I got back on the pitch."
Watson's grandfather, Bill Watson, earned 10 caps for Scotland in the 1970s and later served as chief executive of the SRU. His grandfather and several family members were present in the dressing room when Josh McKay presented Watson with his first Glasgow Warriors cap.
Asked if he had spoken to his grandfather afterward, Watson said: "Not really - I didn't get much of a chance - but I could see how proud he was. That meant a lot.
"There was loads of family there, and plenty of support from back home in Biggar . Seeing my mates there too - it was unbelievable. It really makes a difference."
Reflecting on the dressing room moment, Watson added: "Josh has always been great with me since I came in.
"There were a lot of people in there, but I was more nervous about my speech - I'm not the best speaker - so I just kept it short and sweet."