The move was a real statement of intent and filled Hornets fans with optimism at the start of the Premier League campaign.
However, a very poor first half of the season saw Watford stuck in the relegation battle.
But since Nigel Pearson took over at the club, results have gone their way and things look brighter again.
The mercurial attacker has revealed how he turned down a move to Barcelona before joining Rennes.
"It is true that I chose Rennes over Barcelona and I made this decision because I didn’t want to go to Barcelona to just sit on the bench or play in the B level," Sarr told Watford's official website.
"I wanted to play, I just really wanted to play and that’s why I chose to go to Rennes."
Ismaila Sarr has become a crucial player for the side and his quality really shone through last week when he almost single-handedly ended Liverpool's unbeaten run in the league.
Sarr's two goals and one assist helped Watford undo Jurgen Klopp's formidable Reds at the Hawthrones.
And his fearlessness stems back to making important decisions for the development of his own career.
"I heard many good things about the place," he said.
"I didn’t know anyone there before joining, though. It didn’t matter for me that the club was in France, all I cared about was playing.
"I didn’t feel any pressure from the big transfer fee, I never feel that kind of pressure with transfer prices."
Sarr explained that he used to follow Manchester United fan as a youngster, so the Premier League has always been a dream.
"I used to like Manchester United," Sarr added.
"I always watched the Premier League more than any other league. Everyone in Senegal watches Premier League football."
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