Since taking over in January, Seedorf has led Milan to 10 wins from 20 matches in all competitions, giving them hope of qualifying for next season's UEFA Europa League.
But Sacchi, who guided Milan to two European Cup wins among many successes, said Seedorf still had areas to work on.
"He has shown he has some ideas, but there is a great deal of difference between talking and doing." Sacchi told Radio 2.
"He has managed to achieve some decent results, but his style of play has not always worked.
"In other words, if you want to create a rock band but you only have jazz artists at your disposal, it is a difficult task."
The former Italy coach backed Milan striker Mario Balotelli to go to the FIFA World Cup as part of Cesare Prandelli's squad, if he met the requirements.
"Should Balotelli go to the World Cup? The great players show their personality, their reliability and their professionalism," Sacchi said.
"If Balotelli ticks those boxes, he should go."
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