Juve are being linked with Manchester United striker Zirkzee this month.
Capello told Radio Rai 1: "Zirkzee is the player who sews together the whole game that Thiago Motta would like to play. Vlahovic doesn't have great technique, he's more of a penalty area man. If you don't get him into the penalty area, it becomes difficult for him to play.
"When Motta takes him off, it's because he believes that Vlahovic isn't part of the game system he wants, it's both a tactical and technical choice."
On Motta's progress at Juve, he added: "They have given credence to all the coach's ideas and so far the results are not what they expected. Motta is rightly criticised, even if in the first half against Milan in Riyadh they were the best Juve of this period."