The 20-year-old joined Old Trafford last summer in a deal that will eventually be worth £57.6 million after add-ons.
But the Ligue 1 club’s chief admits he was initially reluctant to approve the forward’s switch to the Premier League.
"If I was hesitant to sell Martial? Yes, because I imagined that the 2015-16 season would be his,” Vasilyev said in an interview with JDD.
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"That's why we had extended his contract in July. Other transfers were more or less expected, not that one.
"There was the money, but also the fact that Anthony had said, 'Mr. Vasilyev this is my childhood dream'.
"If we kill this dream, you can also lose the boy for several months. He sent me his first autographed jersey and thanked me for giving him this chance."