Andre Villas-Boas was sacked in December last year and Tim Sherwood is under pressure with Tottenham likely to finish sixth in the Premier League.
But Ardiles, who made 221 league appearances for the club between 1978 and 1988, said the pre-season inclusions were to blame.
After losing Gareth Bale to Real Madrid, Tottenham spent £107 million on Roberto Soldado, Erik Lamela, Paulinho, Christian Eriksen, Etienne Capoue, Vlad Chiriches and Nacer Chadli.
"When we started this season we were full of optimism, but somehow it has been a very disappointing season," Ardiles told talkSPORT.
"I believe that the players we have bought in this season have been the problem. They are not performing the way we expected. I don't want to name names, but basically all the people we have bought.
"This is a serious, serious problem.
"At the end of the day, whoever the manager is, AVB (Villas-Boas) earlier in the season and later on Tim, there is not a lot you can do with the kind of players we have right now.
"Whoever comes in, or if Tim stays, he has to make a big rebuilding of the team."
