As word spread around Formula One of his death late on Wednesday, confirmed by a source close to the family, tributes poured in from drivers and all whose lives were touched by the wise-cracking neurosurgeon with a love of cigars and whisky.
They included Brazilian Rubens Barrichello, who suffered a huge crash on the same San Marino Grand Prix weekend that claimed the lives of Senna and Austrian Roland Ratzenberger.
"It was Sid Watkins that saved my life in Imola 94. great guy to be with, always happy...tks for everything u have done for us drivers. RIP," Barrichello said on Twitter to 1.7 million followers.
"RIP Prof. Sid Watkins. Sad news for us who stay behind," said Senna's nephew Bruno, who now races for the same Williams team that triple champion Ayrton was with when he crashed on a sunny afternoon in May 1, 1994.
Senna remains the last driver fatality in a Formula One race and much of the credit for the survival of others can be laid at the door of Watkins, known simply as 'Prof' by paddock regulars and 'Sid' by closer friends.
Northern Ireland's Martin Donnelly, Finland's double champion Mika Hakkinen, Austrian Gerhard Berger and F1 team founder Frank Williams all owed much to his quick response and care after life-threatening accidents.
Triple champions Jackie Stewart, another safety pioneer before Watkins came along, and Austria's Niki Lauda counted him as a friend.
"Today the world of motor racing lost one of its true greats," said McLaren chairman and former team principal Ron Dennis, whose cars took Senna to all of his titles, in a statement.
"No he wasn't a driver. No he wasn't an engineer. No, he wasn't a designer. He was a doctor and it's probably fair to say that he did more than anyone, over many years, to make Formula One as safe as it is today.
"Many drivers and ex-drivers owe their lives to his careful and expert work, which resulted in the massive advances in safety levels that today's drivers possibly take for granted."
Watkins was brought in to the sport by Formula One supremo Bernie Ecclestone in 1978, shortly before the death of Swedish great Ronnie Peterson at Monza in September of that year.
The Briton worked closely with former International Automobile Federation head Max Mosley to improve track and car safety, stepping down as medical officer only in 2004 but continuing to play a role at the governing body as first president of its foundation.
In his book 'Life at the Limit: Triumph and Tragedy in Formula One" Watkins wrote of Senna's final days and how distraught the Brazilian was at Ratzenberger's death in qualifying.
Advising Senna not to race, he added: "In fact, why don't you give it up altogether? What else do you need to do? You have been world champion three times, you are obviously the quickest driver. Give it up and let's go fishing."
Senna's reply, the last words he spoke to Watkins, was simple: "Sid, there are certain things over which we have no control. I cannot quit, I have to go on."
Former F1 driver turned broadcaster Martin Brundle also paid tribute to Watkins, tweeting on Thursday morning: "Motor sport has lost a true visionary + character with death of Prof Sid Watkins, 84. Great man, funny too.Saved my left foot being amputated
"Sid W would often prescribe 'a stiff whisky and aspirin' unless your leg was hanging off. His way of saying 'just put up and get on with it'
"Sid was the 'go to' man for health in Motorsport. He helped, saved, or made more comfortable, numerous people who never sat in a racing car."