The former defender questions whether they can be successful without spending money.
The Scottish champions have taken a frugal approach in recent season, opting to try and secure players on the cheap rather than spending big on signigns.
That has seen a number of star players leave the club, with Fraser Forster, Virgil van Dijk and Victor Wanyama in particular all heading for the Premier League.
Their strategy has not been a popular one, and has seen their European form nosedive from the heady days of beating Barcelona under Neil Lennon.
Now, Agathe has hit out at his former club, saying they have to question if they can be successful without spending money.
“They should be bigger and stronger than they ever were,” he told the Glasgow Evening Times.
“They had a chance to make themselves the biggest club in Scotland for a long time, but instead they have tried to not spend so much and you can see what the difference is.”
“They should be out of sight from everyone else by now. You have to ask the question – can you expect to have a successful and winning team at the top level if you do not spend money?”
With Rangers having secured their return to the top flight, perhaps Celtic will change their transfer policy moving forward.
Mark Warburton’s side seen keen on making an impact next season, and that could force Celtic into spending big to ensure they stay on top.
There is no denying that would be best for the Scottish Premiership in general, with that fierce rivalry having been sorely missed.