Brown, who is currently on loan at Rotherham, was coming up through the ranks at The Hawthorns before he left the club for Stamford Bridge in 2013.
Hopcroft told the BBC: “It might turn out a correct decision. At the moment it doesn't look a great decision.
“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.
“He made a decision and his parents made a decision that West Brom wasn't for them at the time.
“I wasn't party to it, but I know really good offers were made.
“He's the one that got away for me. 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 take on 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 a doubt."