Former Spain and Valencia goalkeeper Santiago Cañizares says David De Gea's problem stem from the departure of long-time goalkeeper coach Emilio Alvarez.
The 29-year-old's topsy-turvy season culminated in the FA Cup semi-final loss to Chelsea, in which he was guilty of two errors.
The 48-year-old stated that De Gea was the reason he decided to leave United last summer, citing a 'lack of loyalty' from the Spain international after he signed a contract extension 'behind his back'.
"I believe that goalkeepers mature with age and at a later age than outfield footballers," Cañizares told Tribalfootball.com in an exclusive interview. "Regarding quality goalkeepers, those that know how to look after themselves, their health, to know their bodies, to be in form, they can be around for a long time and perform at an optimum level even past the age of 30.
"What is bad news for De Gea is that Alvarez has gone, someone that David depended on and who kept him in great form. It's not good news for De Gea that Alvarez has gone because a goalkeeping coach can really bring the best out of his 'pupil', which is the goalkeeper.
"It's vital for a keeper to have a great goalkeeping coach from a psychological point of view as well as a coach who knows the goalkeeper's body from a physical point of view and his absolute needs to stay in form.
"Now that Alvarez has gone, it's going to be a big problem for De Gea to resolve. It's vital for a goalkeeper to have a goalkeeping coach with absolute mutual trust and belief in one another. Alvarez left due to a lack of trust with De Gea. Alvarez has gone but De Gea is there so 'no pasa nada'."
De Gea and Alvarez share a private word before United's Champions League clash against Barcelona in April 2019
Despite his troubles this season, Cañizares has no doubts about De Gea's quality and predicts he will stay at Old Trafford for years to come.
However, the 50-year-old adds that De Gea's performances for the Spain national team have not matched his domestic form.
"David De Gea is a magnificent goalkeeper. He'll probably end his career at Manchester United playing many more matches," Cañizares added.
"I believe that he has performed well at Manchester United but when he has played for Spain he has felt a lot of a lot of pressure wearing the jersey and hasn't performed to the best of his ability for the national side."
Asked who should be Spain's number one goalkeeper out of De Gea and Chelsea shot-stopper Kepa Arrizabalaga, Cañizares concluded: "Regarding Spain's manager Luis Enrique, it would appear that Kepa, at the moment, has the advantage regarding being Spain's number one keeper."