Dimitar Berbatov says he still has doubts about Ralf Rangnick's suitability to manage Manchester United.
Rangnick, 63, joined the club on an interim basis until the end of the season but has been unable to make many positive changes to the club during his first few weeks.
United might have only lost once under the German coach, though there have been a number of unconvincing performances against some of the smaller sides in the Premier League.
Former forward Berbatov appreciates that it is still early days in his tenure, but he still doubts whether Rangnick is suited to a job of such magnitude.
"There are a lot of improvements needed still with Manchester United in my opinion," Berbatov told Betfair. "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 practising 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.
"However, look at what Klopp and Tuchel have achieved at their clubs so far. Maybe with a bit of time something similar could happen at Man United with Rangnick."
