John Gregory says Billy Davies left Derby County yesterday because the club simply isn't geared to living with the Premiership big-boys.
Ex-Rams boss Gregory told The Sun: "Billy did a brilliant job to get Derby into the Premiership - but they probably got there too soon. I know the board supported him financially last year to get them up by going out and buying the necessary players in January.
"The players he took into the top flight made up a hard-working, mentally-strong team. But they weren't ready as a group and Derby needed a serious amount of money to give them a chance this season.
"They were always going to find it hard to stay up with the current crop of players, so I'm surprised by the decision to let Billy go. Billy was only given a short period in the top flight, although I guess nothing ever really shocks you in football these days."
Gregory believes the board may have lost faith in their manager's ability to sign the right players.
The ex-Aston Villa and QPR boss added: "You're only really as good as your last purchase.
"The board may well look at the money Billy spent in the summer and question why the likes of Robert Earnshaw have sat on the bench for most of the season.
"Earnshaw cost £3.5million but the decision to leave him out could be interpreted as Billy now thinking he is not good enough.
"In the end, a chairman won't trust you with the chequebook if he thinks you have squandered his club's money."