Seedorf jetted in from Brazil on Wednesday evening, having agreed to take over from the sacked Massimiliano Allegri.
And he arrived in Milan just in time to see his new charges put aside their Serie A struggles to see off Spezia with ease.
Two goals in the space of four minutes from Robinho and Giampaolo Pazzini either side of the half-hour mark pretty much sealed the result before Keisuke Honda added a third two minutes into the second half.
Nicola Ferrari's stoppage-time goal for the visitors was scant consolation.
Napoli had to come from behind to see off Atalanta 3-1 at home, only finishing the job once the visitors had been reduced to 10 men.
Giuseppe De Luca opened the scoring after 13 minutes, but Jose Maria Callejon almost immediately replied.
Napoli did not get their noses in front until Lorenzo Insigne scored in the 72nd minute and, after Mario Yepes saw a second yellow card three minutes later, Callejon's second settled it.
Siena also had to come from behind to beat Catania 4-1 away from home.
The hosts got off to an ideal start when Sebastian Leto scored after four minutes, but Michele Paolucci levelled in the 24th minute and Francesco Valiani put them in front just before the hour mark.
Nico Pulzetti and Lorenzo Rosseti then scored in stoppage time to put a gloss on the scoreline.