The ex-striker passed away at the age of 54 on the weekend, with former Everton and West Ham boss Moyes leading the tributes.
Campbell played for Moyes at Everton in the Premier League, playing over 100 league games for the club.
He also starred for Arsenal in England and Trabzonspor in the Turkish Super Lig.
"First of all I have to say it's tragic news, it really is," Moyes stated on the BBC when asked about Campbell.
"Such a young man, a lot in his life still to do...
"When I first went to Everton, Kevin was already there. He was a huge character in the dressing room, a big player, an incredibly experienced Premier League player at the time as well.
"Really helpful to me when I was starting off, and he was in a really good group at Everton at the time. He was a huge part of it, and it's a real tragedy."