Mane, 33, had a breakout 2014-15 season at Southampton, scoring 10 goals and providing three assists in 30 Premier League games.
Speaking to ESPN, Mane revealed he had a conversation over a potential move to Man United but was put off by the attacking talent they already had.
"Manchester United called me that time, I was talking to Van Gaal,” he said.
“I was talking to Van Gaal that time. Rooney was there. And Di María was there. And ... and (Memphis) Depay was there. When they failed to get me, they bought (Anthony) Martial. Van Gaal called me and he said, 'Mané, how are you? What are you doing?' He said,
"I want you to come to Manchester United. I said, 'Really?' He said, 'Yeah.' I said, 'OK. Now I'm talking to my agent. (Van Gaal said) 'We'll see what is the best, because I know you're a good player and you can help the team, and we can help you also to become better players'.
"And then I said, 'OK, so my question: you have Depay, you have Rooney, you have Di María, you have Van Persie, I think the same. 'So where am I going to play?' That was my question, because me, I want to play.
“He said, 'I know your talent, you're good. But if you do good training, good impression, you will play. But we have another player which is also good'.
"I was not convinced about his explanation. I talked to the coach, but he told me that I will play if I'm good; if not. But at the time, me, I was not ready, I can say. I was young.
“I still needed someone to help me more. And one or two more years to become what I want to be. Because I was still in Southampton; I was not consistent. And then we said, 'OK, we'll see'."