The influential Brazilian international has not played since the dismal 3-0 home defeat by Newcastle - a replay of last season's final - at the start of November that ended United's Carabao Cup defence with a whimper in the fourth round, taken off at half-time with a hamstring issue.
Casemiro, who donned the captain's armband, had only just been returning in that game following an ankle problem sustained while away on international duty with Brazil in October.
Though his form has been notably inconsistent so far this term following an impressive first season at Old Trafford that followed his high-profile £70million switch from Real Madrid in the summer of 2022, the 31-year-old has still been a major miss for United as their wildly inconsistent form has continued under Erik ten Hag.
More pressure was piled on the Dutchman on Saturday, when his team made the worst possible preparation for a crunch Champions League clash with Bayern Munich as they fell to a disastrous 3-0 home loss at the hands of Bournemouth.
Despite their well-documented struggles in Europe and reports of a dressing-room revolt against Ten Hag, United had actually won all their Premier League games in November without conceding a single goal and then brushed aside Chelsea in midweek, with Ten Hag and Harry Maguire receiving the top-flight's manager and player of the month awards respectively before the sobering defeat by the Cherries emphatically halted that resurgence.
United will now hope to have Casemiro back soon, with the club confirming on Sunday evening that he had resumed training outside alone on the grass at Carrington as his team-mates conducted a post-Bournemouth recovery session.
Ten Hag previously said that he did not expect the Brazilian back in action before Christmas, so it will be interesting to see if that timeline has now shifted at all with such a hectic fixture run ahead as the do-or-die visit of Bayern is followed by meetings with Liverpool, West Ham, Aston Villa and Nottingham Forest to close out 2023.
United have seen the likes of defenders Luke Shaw and Jonny Evans return from injury in recent weeks, though are still without Lisandro Martinez, Christian Eriksen, Victor Lindelof, Tyrell Malacia and the unavailable Jadon Sancho.
Mason Mount and Amad Diallo are both back in training but have yet to play after injury, while Raphael Varane was an unused substitute against Bournemouth after a back issue.
Captain Bruno Fernandes, meanwhile, is banned for the clash with fierce rivals Liverpool at Anfield on Sunday after collecting his fifth yellow card of the season for dissent against Bournemouth, triggering an automatic one-match suspension.
That ban only applies to domestic competition however, with Fernandes free to face Bayern on Tuesday as United face a must-win game in their bid to reach the last 16 of Europe's elite club competition.