Fellaini wants £2m in damages for New Balance boots that rubbed
It's claimed the "defective boots'' had an impact on his performance.
United's Belgian international midfielder says sportswear giant New Balance gave him "defective quality'' boots which had to be "steamed and stretched'' before he could wear them.
A High Court judge might have to examine a series of WhatsApp messages if asked to rule on a row over the boots.
An investment company which represents the midfielder has sued New Balance and wants around £2 million in damages.
Rosalina Investments says "defective boots'' had an "impact'' on 30-year-old Fellaini's performance and caused a "loss of enjoyment'' and "inconvenience''.
New Balance bosses have hit back at the claims - saying they gave him a "bespoke" pair of replacements when Fellaini said he was "experiencing discomfort''.
Details of claims and counter claims have emerged in papers lodged by lawyers at the High Court in London.
Paperwork shows that evidence is contained in WhatsApp messages sent between Fellaini and New Balance staff.
Barrister Ruth den Besten, who represents Rosalina Investments, has outlined Fellaini's complaints: "The football boots supplied by New Balance were of poor quality and caused considerable damage to Mr Fellaini's feet,'' she said.
"In order that the boots fit Mr Fellaini, they were required to be steamed and stretched by the Manchester United FC's assistant kit man.''
She added: "Mr Fellaini repeatedly communicated his frustration with the poor quality of the boots.''
But Ian Mill QC, who represents New Balance, said the sportswear firm had solved the problem: "The player orally informed Mr Simon Crawford of New Balance that he was experiencing discomfort when wearing certain boots,'' said Mr Mill.
"New Balance therefore arranged for the player to be provided with bespoke replacement boots.''