Boban, now involved with UEFA, was a Milan director when they signed Tonali from Brescia.
He told Sky Italia: "Milan sold Tonali, their best Italian player and very attached to the shirt. Seeing him away from Milan is a shame, these things have to be paid for. To come to Milan he refused Juventus who had offered €42m to Brescia: these are the values that must never be lost."
On Milan's Champions League defeat to Liverpool last night, Boban continued:"Big problems for Milan. There is no construction of play, the two central defenders are very slow and awkward, they don't have the courage and personality to play the ball forward.
"In the middle of the field there is no one who takes responsibility to do more concrete, important things, to get the ball to the wingers or the attackers. Then for an hour they play on the right? I don't understand. Never seen a ball on the left for Theo or Leao.
"It's not logical to play for an hour only on the right, so it was probably a choice. The most worrying thing was certainly the lack of reaction at 3-1. Only Abraham did something good, but they're obviously missing a leader. And there's a lot to build on."