Mourinho signed Pogba in 2016 for a then world record fee.
But the Portuguese coach struggled to consistently get the best out of Pogba.
And throughout 2018 Pogba and Mourinho clashed over matters on and off the field.
At the time of Mourinho’s sacking by Manchester United in December, Pogba had been dropped to the bench.
But since the Ole Gunnar Solskjaer’s appointment, Pogba has four goals and three assists for United.
And Mathias, who plays for French side Tours, told French radio station RMC that Mourinho was at fault for the midfielder’s struggles.
“The problem was Mourinho, right down the line,” Pogba’s brother said.