Milan lost 2-1 against Torino on Saturday and are now eight points below the top four with a game in hand.
Conceiçao was appointed at the end of December to replace his compatriot Fonseca and had a strong start in the Supercoppa, beating Juventus and Inter to lift the first trophy of the season in Italy.
However, his record in his first 15 games in charge is worse than Fonseca’s in his last 15 games at San Siro.
Conceiçao has collected eight wins, three draws and four defeats across all competitions, averaging 1.8 points per game.
In the last 15 games before his sacking, Fonseca had averaged 2.06 points per game (31 points), securing nine victories, losing two games and drawing four.
Conceiçao’s Milan lost to Juventus, Dinamo Zagreb, Feyenoord and Torino, while Fonseca’s only two defeats came against Napoli and Atalanta.
Milan return to the pitch on Thursday in a rescheduled Serie A game away at Bologna, one of their direct rivals in the race for a European spot.