Murray has been outstanding for Brighton this season.
Parish told the Croydon Advertiser: "The fact that Glenn is not at this club is a massive regret for me. We made a mistake, simple as that.
"He did have a cruciate injury and I was told by managers he wasn't good enough and that he wouldn't make it. In the Premier League he didn't do this and he didn't do that.
"It's difficult. They are your football experts. I don't think they were doing it out of spite, that they didn't like Glenn - that's actually what they believed at the time.
"Maybe they hadn't watched him enough. Sometimes you don't realise people are good enough until they are given the opportunity."
Murray has scored four goals in the Seagulls' last four Premier League games.