Two weeks ago, Zayn quit One Direction after five years. His announcement came just a week after pulling out of their world tour after being signed off with stress.
"I knew for a while he was thinking about, you know, the future," Simon told BBC Newsbeat. "It did come as a bit of a shock, but I have to look after him and look after the boys."
Music mogul Simon, who formed One Direction as a group after the boys auditioned as soloists on The X Factor in 2010, said no-one should be forced into doing something they don't want to do.
Remaining 1D members - Niall Horan, Liam Payne, Louis Tomlinson and Harry Styles - said they respected Zayn's decision in a statement, but told fans they would continue as a four-piece.
Simon said the rest of the boys became "united and strong" after getting over the shock. "They put on some incredible shows afterwards," he added. "The future's bright, I think."
It is thought that Zayn will carve out a career as a solo artist, and it seems like Simon is supportive too, after saying as well as the boys, "Zayn will have a great career as well".
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