The Republic of Ireland striker was sold to Hull City for £7m days after Mel's appointment in January.
Long will face his former club for the first time on Saturday when the Baggies visit the KC Stadium, with both clubs fighting for points to aid their Premier League survival chances.
Mel, though, has confessed that he had little say in the 27-year-old's sale to the Tigers, claiming that he was effectively out of the door before he arrived at The Hawthorns.
"Before I arrived here, Long was out. It was not my problem," he told reporters.
Asked if he would have liked to have kept Long at West Brom, Mel added: "Of course - Romelu Lukaku (who was on loan with West Brom last season from Chelsea) too.
"But it was not possible. My job began on January 14, and Shane Long was at Hull. Shane Long is a good player, a good striker, but we have other players."
Long scored 22 goals in 87 appearances for Albion after joining from Reading in 2011.