Despite Watford kicking off, Long rapidly closed down an attempted long ball from Craig Cathcart, before dinking smartly over goalkeeper Ben Foster.
"You know that we depend on the results, at different times, so making medium-long-term plans for me does not make sense".
It wasn't enough to secure victory for the Saints, however, as Andre Gray smashed home in the final minute to earn the Hornets a share of the points. Subscribe to our SMS sports service by texting "SPORTS" to 22840.
Former Socceroo Damian Mori netted after a scarcely believable 3.69 seconds for Adelaide City in a 2-2 National Soccer League draw against Sydney United in December 1995 - a strike that held the world record between 1995 and 1998.
And Long's goal helped Southampton to another vital point in their quest for safety.
The goal was officially clocked at 7.69 seconds into the game at Vicarage Road, quicker than Ledley King's previous mark of 10 seconds.
Van Dijk hoping for Liverpool wins, title triumph and nice weather!
The Sky Blues have four remaining games - Manchester United tomorrow, Burnley, Leicester City and Brighton & Hove Albion. It's a bit of a waste of energy, isn't it, willing the ball into the goal and things like that.
King had scored 9.82 seconds into a game against Bradford City the 2000/01 season.
A goal after seven seconds to set the tone and give your team an advantage is the best any team can really ask for.
A straightforward pass from the kick-off by Roberto Pereyra to Craig Cathcart ended with the defender striking the ball against Shane Long, who skipped forward to chip over the advancing Ben Foster.
The Saints could have scored more with Nathan Redmond and Ryan Bertrand both hitting the woodwork.
Watford moved above Everton into seventh place to keep up their hopes of qualifying for the Europa League.