Spaniard Mel's appointment as head coach and successor to the sacked Steve Clarke was announced by the Baggies on January 9.
Long was still a West Brom player at that point and completed a full 90 minutes for them on January 11 as they lost 1-0 at Southampton - a game for which they still had caretaker boss Keith Downing in charge while Mel watched from the stands.
Prior to Albion's next fixture, Long's transfer to Hull was then confirmed on January 17, and the Republic of Ireland international is set to line up against his old club on Saturday when they visit the KC Stadium in the Barclays Premier League.
Speaking at his pre-match press conference, Mel said of the frontman: "Before I arrived here, Long was out. It was not my problem."
Then, asked if he would have liked to have kept Long at West Brom, Mel said: "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."
Mel does not think Long will feel he has a particular point to prove against his former employers in Saturday's match.
"I think not, because for professional footballers, it is normal to change job," Mel said.
"And besides, the most important thing in football is the fans and what they feel for the club.
"Shane Long is a good player, a good striker, but we have other players."
Mel has been particularly keen to pay tribute to the West Brom supporters and the way they have been towards him since he joined the club.
Albion, currently three points above the relegation zone in 16th place with nine more games to go, only registered their first win under the 51-year-old last weekend in what was their eighth attempt as they saw off Swansea 2-1 at the Liberty Stadium.
Some Baggies fans brought Spanish flags with them to the game as a show of support for Mel.
And regarding that, he said: "It is difficult for me to explain how I felt in this moment.
"I think West Brom is a family club, and for me, this is very good.
"I have had eight matches here and only one win, but the fans have always been very, very kind with me. It is incredible.
"I think West Brom is a family altogether - the staff, the fans, the players. And this is important for the future."
Defender Jonas Olsson has joined West Brom's list of unavailable players for the clash against 13th-placed Hull, who are two points better off than Albion.
With vice-captain Olsson beginning a two-game ban after reaching 10 yellow cards for the season and first-choice skipper Chris Brunt already out with a knee problem, Gareth McAuley is taking the armband this weekend.
Diego Lugano (knee), Billy Jones, Claudio Yacob (both hamstring) and Morgan Amalfitano (knee) all remain sidelined.