Hartson has signed a two-year contract at The Hawthorns after Albion paid £500,000 for the Welshman who had a year left on his contract at Parkhead.
But the 31-year-old revealed he had received verbal agreements he would be spending the 2006-07 campaign with the Hoops.
“It was Celtic’s decision to listen to offers for me,” said Hartson. “I wasn’t in Gordon Strachan’s plans for the new season and that’s fine.
“Having heard in the summer they would accept a bid for me, it did take a couple of days for it to sink in that I might actually be leaving.
“I shook hands with several people at the club about me staying until the end of my contract.
“But things change in football – I know that – and I haven’t got a bad word to say about Celtic or their fans.
“They paid me a lot of money, gave me good contracts and I left as a member of a championship side, not under a cloud.”