Albion’s head of academy recruitment claims Brown, who was 16 at the time, may have made the wrong move.
“It might turn out a correct decision. At the moment, it doesn’t look a great decision,” Hopcroft told BBC WM.
“I absolutely loved Izzy Brown. I thought he was the best of all the players that have left here.
“He could have gone on to play a similar number of games as Saido Berahino. He could have made his 100th appearance for West Brom and be in a better position than he is now.
“For whatever reason he chose not to go down that route. He chose to go down another route with a bigger club.
“He made a decision and his parents made a decision that West Brom wasn’t for them at that time. I wasn’t party to it, but I know really good offers were made.”
The England Under-17 international became the second youngest-ever Premier League player when he made his Albion debut at 16 in May 2013, but moved on later that summer for an undisclosed compensation fee, decided by tribunal.
“He was the one that got away for me,” said Hopcroft. “The manager at the time, Steve Clarke, was ready to make him an integral part of the team.
“We had a potential £20-30-40 million player that the fans would have loved, full of flair and skill. He could on take six players from his own half and score.
“I was bitterly disappointed, as was everyone in the Academy. He’s still potentially a world-class player, the best we’ve lost, without a shadow of doubt.”