Milan were humiliated 5-2 at San Siro by Sassuolo on Saturday to leave the defending champions without a win in their last six matches.
Next up is the Serie A derby against fierce rivals Inter Milan next Sunday.
It was the first time in Milan's history that they have conceded four goals in successive Serie A matches after losing 4-0 at Lazio midweek.
It was also the first time since 1997 that Milan have conceded five goals in a league match at home.
Milan also crashed out of the Italian Cup against 10-man Torino and lost the Italian Super Cup 3-0 to Inter.
At the start of the year, Milan were second in Serie A and within five points of leaders Napoli. They have now dropped out of the Champions League places - after Lazio drew against Fiorentina 1-1. The Rossoneri are 12 points behind leaders Napoli.
"At the moment, as soon as we suffer a blow we can't manage to react," Pioli said.
"I know that there's not the same serenity as we had before, so we have to show that we can still have a good campaign and get into the Champions League … that will be our title."
Milan director Paolo Maldini had laughed off suggestions midweek that Pioli's job could be on the line, just eight months after leading the team to their first Serie A title in 11 years.
Sassuolo moved up to 16th, eight points above the relegation zone with their first win since mid-October.
Domenico Berardi was again a thorn in Milan's side, with three assists and a goal.
Milan actually started the stronger side and Olivier Giroud thought he had given his team the lead in the eighth minute, but it was ruled out for offside.
And then the team collapsed.
Berardi set up Grégoire Defrel for the opener in the 19th minute and then provided another assist for Davide Frattesi to smash past Ciprian Tătărușanu just three minutes later.
Milan managed to pull one back two minutes later when Giroud headed in Davide Calabria's cross.
However, Berardi headed in a corner on the half-hour mark to restore Sassuolo's two-goal advantage.
Things went from bad to worse immediately after halftime as Calabria fouled Armand Laurienté in the opening minute of the second half and the Sassuolo forward converted the resulting penalty.
Matheus Henrique capped off Sassuolo's memorable match in the 79th minute, shortly after coming on, with his first Serie A goal - again set up by Berardi.
Divock Origi scored a consolation for the hosts two minutes later.
JJuventus welcomed back Dušan Vlahović after three months and had Paul Pogba on the bench but that was the only good news as the Bianconeri lost 2-0 at home to Silvio Berlusconi's Monza.
Patrick Ciurria and Dany Mota scored in the first half for Monza.
Monza, who became the first team in history to beat Juventus home and away in their debut Serie A campaign, leapfrogged the Bianconeri into 11th spot.
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