The news came as last night Pedro told Barcelona he wants to leave the European champions.
United want to sign Barcelona forward Pedro as a replacement for Angel di Maria, who joined Paris St Germain last week.
The 28-year-old is regarded as one of the best attackers in Spain, but has become frustrated at his lack of first-team football following the arrival of Luis Suarez and Neymar, who have formed a deadly three-man attack at the Camp Nou with Lionel Messi.
Former Barcelona striker Stoichkov thinks Pedro would be making a huge mistake if he moved to Old Trafford, however, because he believes Van Gaal is a poor coach.
The Bulgarian clashed with Van Gaal during the Dutchman’s first spell in charge of Barcelona and he clearly does not have a high opinion of the 64-year-old.
“I would never be under the orders of this Dutchman (Van Gaal),” Stoichkov said.
“I would never leave if I was Pedro because if he goes there he would no longer be a footballer. I hope Pedro listens to me and he tells them (United) ‘I am staying at Barcelona’.”
Van Gaal sold Stoichkov to CSKA Sofia 17 years ago, but the pain of leaving the Spanish giants is clearly still raw for the Bulgarian.
“He makes players suffer,” said Stoichkov.
“He is destroying all the players because he is so mediocre, players have left because of him.
“He destroys clubs, he did it with Barcelona, and after they take years to build.”
Meanwhile, Peter Schmeichel has criticised the handling of David de Gea’s contract situation.
The Real Madrid target watched from the stands as United began their season with a 1-0 win over Tottenham on Saturday.
Manager Louis van Gaal selected Sergio Romero ahead of De Gea as he felt the 24-year-old would not have been able to focus properly on the match after a summer of speculation regarding his future.
Schmeichel admits the situation is far from ideal and should have been resolved before the season began.
“If the player is not happy; his agent, himself and the club that want him should have sorted this a long time ago,” the former United goalkeeper said. “It’s sad that it’s dragged into the season.”
Schmeichel wants De Gea to snub a move to the Spanish capital, but concedes there is little United can do if the player wants to leave.
“I’d really like De Gea to stay,” said Schmeichel.
“Two years running he’s been player of the year. You don’t let players like that go but if he wants to go, let him go and let Manchester United move on.”
If De Gea does leave, Schmeichel believes Tottenham keeper Hugo Lloris is the man United should get.
“If De Gea goes I would love Lloris to come to Manchester United. He has exactly the style of play and he is proven in the Premier League.”
Last night De Gea was later included in United’s Champions League squad for the two-legged play-off against Club Brugge. However, even if he appeared in either of the matches he would still be eligible to represent another club in the group stages of the competition this season, according to Uefa rules.
Elsewhere, Michael Carrick has insisted he never contemplated leaving United this summer.
Although Carrick signed a new deal late in the spring, the 34-year-old was linked with a move to Newcastle and Inter Milan after United signed Bastian Schweinsteiger and Morgan Schneiderlin.
However, the thought of leaving Old Trafford never crossed the midfielder’s mind, it seems. “It (leaving) is not something I thought about at all. It has never been an issue.”