A dismal first season at Stamford Bridge for the Italian has seen Chelsea suffer their heaviest defeat for 28 years - a 6-0 thrashing at Manchester City last weekend that came just weeks after a 4-0 loss at Bournemouth.
Amid claims Sarri has lost the support of his players, the former Napoli boss held clear-the-air talks this week in a bid to convince the squad to stick with him.
Sarri has accused Chelsea's stars of being hard to motivate, questioning their mental strength and claiming they don't understand even the basics of his game plan. The players are said to be unhappy with the Italian's training methods and refusal to adapt his tactics to the squad he has at his disposal. Cesar Azpilicueta, Marcos Alonso and N'Golo Kante are all being asked to perform roles that are ill-suited to their strengths.
While 'Sarri-ball' was a big hit with Napoli, the Blues have struggled for cohesion in midfield and lacked cutting edge up front.
Chelsea have been beaten in seven of their last 22 games but they are still in the Europa League after defeating Malmo in the last-32 first leg. They also face Manchester City in the League Cup final. "We know it's a crucial moment," Sarri's assistant, Gianfranco Zola, said. "We have some very important matches ... It's very challenging."
Responding to the sense of gloom around Chelsea, Zola called on his squad to pull together.
United were beaten by Chelsea in last season's final, but much has changed at Old Trafford. Jose Mourinho was sacked in December and Ole Gunnar Solskjaer had overseen a remarkable turnaround as interim boss.
Solskjaer went unbeaten in his first 11 games in all competitions, lifting United above Chelsea into the top four. But their momentum stalled when Paris Saint-Germain won 2-0 in the Champions League last 16, first leg at old Trafford.
United have also been hit by injuries to Anthony Martial and Jesse Lingard.