Manchester United 's players could be struggling to adapt to Ralf Rangnick's tactical methods according to Dimitar Berbatov.
Rumours of discontent continue to filter out of the Red Devils' camp.
Rangnick was hired as Ole Gunnar Solskjaer's successor, but performances have not improved despite the German's sky high reputation.
Players are said to be struggling with aspects of training as the interim boss attempts to get his ideas across.
Berbatov, who previously played in the Bundesliga before moving to England, accepts that the German school of coaching can be boring at times.
The former United man believes that coupled performances, which have continued to be below par, can lead to a negative vibe around the group.
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He told Betfair: "There are a lot of improvements needed still with Manchester Untied in my opinion, I'm still not convinced with Ralf Rangnick, he has never been in charge of a really big team like United, or Barcelona, Bayern Munich, the teams that are the biggest in world football.
"Even with all his accomplishments in football and the respect that Jurgen Klopp and Thomas Tuchel have for him, this is the biggest challenge for Rangnick, to know how to succeed at a big club and so far it's not quite worked how people expected it to.
"I have read the rumours that some players are a bit unhappy at the club.
"Having been a student of German football and having played there for five years I know that sometimes it takes a while to get used to new methods of training and practicing and knowing what the new coach wants tactically, and maybe the training can be a bit boring from time to time.
"These types of things can cause a bit of unhappiness and when you put that together with some not so convincing performances then you have this type of situation."
Rangnick saw his side see off Aston Villa last time out to advance to the fourth round of the FA Cup.
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Their display again left a lot to be desired as the visitors enjoyed plenty of success, only to continually fluff their lines.
United's last league outing also saw them suffer defeat at Old Trafford against Wolves as they continue to chase the top four and Champions League football.
The German boss is also dealing with a host of players considering their own futures having grown frustrated by their lack of minutes.
Eric Bailly, Donny van de Beek and Dean Henderson have all been linked with the exit door.
