Koeman's best league result against Spurs was a 2-2 draw in April last year, with former Southampton boss Mauricio Pochettino's men prevailing on the two other occasions the sides have faced each other in the domestic top-flight.
"The gaffer said in the last couple of years there's only two teams we didn't beat -- Man City was one and Tottenham was the other, so that's one hurdle over," Long told the club's website (www.saintsfc.co.uk).
"Now we've got to go to Tottenham and try to repeat the same result."
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The 29-year-old has struck 10 league goals this term as the Saints push for European football, but is wary of the threat posed by second-placed Tottenham, whose league title hopes disappeared after drawing with Chelsea on Monday.
Tottenham are unbeaten in their last eight league games and victory would guarantee runners-up spot.
"We're very confident," Long added.
"It's another tough game for us, but we're playing good football and there's no reason we can't go there and get the three points."
Southampton are seventh in the table and end their campaign at home to Crystal Palace on May 15.
(Reporting by Nivedita Shankar in Bengaluru; editing by Martyn Herman)
