Newcastle United legend Kevin Keegan blamed owner Mike Ashley for the club's relegation, telling BBC Sport: "It is mismanagement from the very top."
Newcastle's return to the Championship was confirmed on Wednesday after arch-rivals Sunderland beat Everton 3-0 to move three points clear of the relegation zone.
Keegan, who had a brief and unhappy spell under Ashley when he returned to Newcastle as manager in 2008, said the owner bears the responsibility for the club's failure this season.
"You have to say Mike Ashley hasn't handled the decision-making very well or he's given the responsibility to people and they haven't handled it very well," the former England boss said.
Newcastle appointed Steve McClaren last summer and spent heavily to reinforce the squad, bringing in the likes of Georginio Wijnaldum, Aleksandar Mitrovic, Chancel Mbemba and Florian Thauvin for significant fees, after having endured a dismal end to the 2014-15 season under caretaker John Carver.
They are reported to have taken their net spend for the season to over £70 million in January when they brought in Jonjo Shelvey, Andros Townsend and Henri Saivet, as well as loan signing Seydou Doumbia, and then brought in boss Rafa Benitez with 10 games remaining.
Keegan recognised that the club had invested in the squad but said there had not been enough emphasis on bringing in established top-level players.
"It is mismanagement from the very top," he said. "They bought players for the future but sometimes maybe you have to buy players for the present.
"It's all right having a few good young players from France but can they play in the Premier League now?
"If they can't, don't buy them or certainly don't play them. A few of them have found it very tough and the confidence goes and then even good players start to look pretty poor."
Keegan, who enjoyed a successful two-year spell with Newcastle as a player, guided the club out of the second tier in 1992-93 during his first season as manager, and they remained in the top flight until 2009.
Chris Hughton's side then secured the Championship title in 2009-10, and Keegan is hopeful that they can make a swift return again this time around.
"They are going to have to go down and regroup," he said. "That's not going to be easy. You make mistakes in football and you get punished, especially the Premier League now. I think they'll bounce back."
Faustino Asprilla, who played under Keegan at Newcastle in the 1990s, was also critical of the owner.
@NUFC Welcome to the championship; you guys worked very hard to get there! pic.twitter.com/hc5fPujnCa
- Faustino Asprilla (@TinoasprillaH) May 11, 2016
"Newcastle United have been very poor this season," he told the Evening Chronicle newspaper prior to the confirmation of their relegation.
"For me the problems started with the owner with his failure to invest in consecutive transfer windows, then the board who appointed Steve McClaren. He was never going to be able to handle such a big club.
"The board were also to blame for wasting the money that Mike Ashley gave them. Players such as Mitrovic and Thauvin have just not been good enough."
The Evening Chronicle used its front page on Thursday to urge managing director Lee Charnley to do everything in his power to convince Benitez to stay on at the club next season.
Benitez, who was sacked by Real Madrid in January, has a break clause in his contract that means he is free to leave the club following relegation and had been coy about his intentions after the 0-0 draw with Aston Villa at the weekend.
NUFC fans, what would you say to Rafa Benitez to get him to stay? Tell us: #DearRafa https://t.co/cyj614vBB6 pic.twitter.com/odkIGoGEq8
- The Chronicle (@EveningChron) May 12, 2016
Asprilla said Newcastle's hopes of securing promotion next season depend on the former Liverpool and Napoli boss remaining at the club.
"It depends on if Rafa stays, and that is a big if," the former Colombia striker said. "If he leaves the club will suffer.
"I don't think we have the players to get us out of the Championship although I hope I'm wrong, but unless things change at the top and on the pitch then my old will be down for a few years."