Samuel Eto'o has issued a warning to former boss Jose Mourinho, telling him that football is evolving and "you can't defend continually".
The Portuguese has endured a difficult start to the Premier League season with Manchester United currently 10 points off the top four ahead of Sunday's clash at Everton.
While United have reached the semi-finals of the EFL Cup and are on the verge of securing a place in the last 32 of the Europa League, they are yet to truly impress.
Mourinho’s career has been mixed over recent years. He won silverware at Porto, including the UEFA Cup and Champions League, guided Chelsea to successive league titles and won an unprecedented domestic and European treble at Inter, but his spell at Real Madrid was less glamorous, as he lifted La Liga just once in three years.
The 53-year-old won the Premier League upon his return to Stamford Bridge, only to be sacked in December 2015 after a disastrous start to the season.
He has since been charged with leading United back to the top level of English and European football, but has come under criticism already.
And Eto'o, who played for Mourinho at Inter and Chelsea, believes the Portuguese is at risk of being left behind if he does not develop his methods.
"Jose has changed a lot," he told Telefoot. "I discovered someone exceptional when I came to Inter. At Chelsea, it was already a bit difficult.
"He asked for three years to relaunch Manchester United, but you have to get your team playing, you can't defend continually. Football isn't like that anymore.
"Let's be clear: I've nothing against Jose. He is able to pick someone, tell him 'you are the best' and he'll have a very good game. But football is evolving."
United have won just one of their last seven Premier League encounters heading into the game at Goodison Park.