The Spain international is a transfer target for United but Stoichkov feels he would be better off staying with Champions League winners Barcelona.
Stoichkov barely featured under Van Gaal when the pair worked together at the Nou Camp and he was sold by the Dutchman in 1998, bringing an end to his second spell with the club.
The former Bulgaria international striker was scathing about Van Gaal when asked about him by Spanish radio station Onda Cero.
“I would never be under the orders of Van Gaal,” Stoichkov is reported as saying. “He’s mediocre. He destroyed Barca and is destroying Man Utd.
“With what he has done to goalkeeper [Victor] Valdes he has shown he is a bad person. Hopefully Pedro listens to me and doesn’t leave.”
Barcelona boss Luis Enrique hinted Pedro’s future would be sorted out soon when he was asked about the transfer before the club’s Super Cup final clash with Sevilla in Tbilisi.
He said: “I would like to keep all my players through the whole season. But we have talked enough about Pedro. The solution will arrive soon.”