We were the only club that was in for Mane at the time we went in for him while Van Gaal was here but our penny pinching stopped the move hence there was no way Mane could have chosen Liverpool over us.
Maybe we went back for him the following season after Liverpool made their interest known, which isn't surprising considering Mourinho's love for hijacking players on the verge of moving to a rival club. He did it with Willian who almost moved to Spurs. Likewise, Pedro, Mikel Obi while he was at Chelsea and while he was with us, he hijacked Lukaku, Fred, Sanchez and almost did with Kante in the last minute hence same role could have played out with Mane on his move to Liverpool but while Van Gaal was here, we were the only club in for him but the move broke down because we couldn't meet Southampton's asking price.
As for Keane and Rashford, this is even lame because left for any other coach, none of these players would have been promoted in the first place. Besides, Rashford was a winger and was yet to become an established player for the u23 team. The fact that Van Gaal even promoted him says a lot about Van Gaal's boldness and trust in youth. I mean this was the same man that benched Ribery to make way for a young Muller. I don't know why he didn't do same with Rooney but I understand perfectly considering the fact that Rooney was English, had just signed a long contract and the media as well as our old guards loved him and were making excuses for his poor form, even blaming Van Gaal, it was quite difficult to drop him at the time.