There was, however, a show of support for the Portuguese coach as Zlatan Ibrahimovic and club owner Gerry Cardinale attended training at Milanello on Friday.
Tomori suffered a lesion to the medial flexor muscle in his right thigh during Wednesday's 4-2 defeat at San Siro. The defender had been forced off at half-time after picking up the problem, and subsequent examinations confirmed the extent of the injury.
A further medical assessment is scheduled in two weeks, but the initial expectation is that the Englishman will be unavailable for approximately a month. He will therefore miss Saturday's derby against Inter, with his absence adding to the problems Amorim has faced in keeping his defensive options available.
Tomori had been enjoying an increased role in recent weeks, starting four consecutive matches and contributing an assist during the Bologna game. He had accumulated 257 minutes across five appearances this season, making the timing of the injury particularly frustrating for the Milan coach.
The setback comes during an already difficult period for Milan's medical department, with several players having been unavailable. The expected returns of Adrien Rabiot, Mario Gila and Diego Moreira offered some encouragement ahead of the Inter fixture, but Tomori's absence leaves another gap to address.
There was also a significant presence at the training ground as AC Milan prepared for one of the most important matches of their season. Ibrahimovic, now a senior adviser to RedBird, joined Cardinale at Milanello to watch Amorim's final training session before the derby.
The former striker's return attracted attention because his involvement in the club's day-to-day football operations has been questioned in recent months. His previous visit to Milanello had been on September 14, two days after Milan's 2-2 draw against Lazio and two days before their 2-0 defeat to Benfica in the Europa League.
Ibrahimovic's latest appearance, alongside Cardinale, was intended to show the backing of the club's ownership and senior leadership for the players and coaching staff. Both were present as the squad completed its preparations for the meeting with Inter.
The derby provides an immediate opportunity for Amorim's side to respond after the disappointment against Bologna, but the coach must now plan without Tomori. With the defender facing a spell on the sidelines, Milan's available defensive resources will be under further scrutiny as they approach a crucial run of fixtures.