The Senegalese attacker opted to join Jurgen Klopp at Anfield instead.
"I couldn't believe it," he said about joining Liverpool. "Some teams wanted me – Manchester (United), the wrong team wanted me.
"Of course, I spoke with Van Gaal, and they even made an offer. But for me it was not the right club, it was not the right moment. To feel this - the right moment, the right coach, the right club."
On finally meeting up with Klopp at Liverpool, the 26-year-old added: "We know it before he came here because you see on TV, it's exactly how he is as a person.
"He's always laughing but he can be hard in training."