Michael Ballack thinks Jose Mourinho is making a big mistake in his handling of Chelsea midfielder Juan Mata.
As reported by Portuguese newspaper A Bola, Ballack admits he can't understand why Chelsea are struggling to get the best out of their forwards.
Samuel Eto'o - the hat-trick against Manchester United withstanding - Fernando Torres, and Demba Ba have struggled for goals this season.
Attacking midfielder Eden Hazard is currently the club's top scorer this season.
And Ballack, who played for Chelsea between 2006 and 2010, believes Mourinho is to blame for the strikers' scoring woes.
"I cannot understand why Mourinho is having problems when it comes to getting the most out of his strikers," Ballack said.
"I also think he's making a mistake in the way he uses Mata."
The Spain international has been heavily linked with a Stamford Bridge exit this month - United are thought to be interested.
He's made 13 appearances for Chelsea in the Premier League this season, but Eden Hazard, Willian and Oscar have often been preferred to him in the starting lineup.
Former-Germany star Ballack made over 100 appearances for Chelsea in his four years at the club, winning three FA Cup trophies, one Premier League and one League Cup.