Hernandez left the club in the summer, having previously been on loan at Real Madrid, and explains why he left Old Trafford on a permanent deal.
Van Gaal has been under a lot of pressure this season as United tumbled down the table midway through the campaign, and they are four points behind neighbours Manchester City in the race for a Champions League place. Van Gaal has also seen his side eliminated from the Champions League at the group stages and knocked out of the Europa League by Liverpool but has an opportunity to win silverware this season in the FA Cup.
Hernandez told Sky Sports: "He is a really good manager to be honest. He has his own ways like all the managers in the world.
"You will not find the new Sir Alex Ferguson in the next 50 or 100 years because he was unique. He was special.
"Van Gaal can do it but in football, time and results are going to speak about if he is the main man or if he is not."
Some have suggested that Hernandez should not have been sold, especially given his goal-scoring exploits for Bayer Leverkusen in the Bundesliga, while United have struggled to find the net regularly this season. The Mexican has managed to score 25 goals from 40 games in all competitions so far.
But he was told that his opportunities to play for United would be limited, with van Gaal saying that captain Wayne Rooney would be his first choice forward.
He added: "He just told me that my chances to play would be very hard. I appreciate that he was very honest since the beginning.
"When I left, he told me my chances and my opportunities were going to be less because his main striker was going to be Wayne Rooney.
"If he wasn't fit for example, I was going to play but when he was ready he would play."
Hindsight is a wonderful thing as Hernandez has proved in Germany that he is a very good goalscorer, and he showed it during his time at Old Trafford with his knack of popping up at the right time with an important goal for the side.
Van Gaal's future at the club is one that is constantly under the microscope as rumours about former Chelsea boss Jose Mourinho succeeding him at the club simply will not go away, even if the Dutchman were to deliver silverware this season - with Saturday's FA Cup semi-final against Everton the biggest game of his tenure.