The 27-year-old had been Cameroon's undisputed number one since 2018, helping his country place third at last year's Africa Cup of Nations following a doping suspension.
Onana unsurprisingly travelled to Qatar to represent Cameroon at the 2022 World Cup, but after their opening 1-0 loss to Switzerland, he left the camp after a falling out with Rigobert Song, allegedly over the latter's tactics.
Devis Epassy started in goal for Cameroon's final two group-stage games with Serbia and Brazil, and Onana's fracas with Song was serious enough to see the goalkeeper call it quits with his national team,
However, Onana's retirement lasted all of 10 months, as he made his return to the Cameroon setup for September's AFCON qualifiers, keeping a clean sheet in a 3-0 win over Burundi this month.
The Indomitable Lions' final victory saw them seal first place in qualifying Group C to ensure a spot in the AFCON Finals, which take place from January 13 to February 11 next year.
As a result, Man United risk being without their first-choice goalkeeper for the duration of the tournament, and The Mirror claims that Ten Hag is seeking instant clarification from Onana regarding his decision.
Upon his arrival from Inter Milan in the summer, the former Ajax stopper apparently swore off an international comeback to Ten Hag, who is hoping that the Burundi win was a one-off and that he will not go to the upcoming AFCON.
However, should Onana indeed jet off to Ivory Coast, fellow new signing Altay Bayindir would be forced to act as the last line of defence for a few weeks, during which Man United face Tottenham Hotspur, Wolverhampton Wanderers, West Ham United and Aston Villa in the Premier League.
The fourth round of the FA Cup is also due to take place on the final weekend of January, while the second legs of the EFL Cup semi-finals are due to be played the week beginning January 22.
Furthermore, should Cameroon progress far in the tournament, Man United could be without Onana for the first leg of any Champions League last-16 tie, which begin on February 13, 2024.
The 27-year-old has experienced a challenging start to life at Old Trafford, keeping just one clean sheet in his opening five Premier League games and shipping 10 goals.
Onana is not the only Man United player in line for an AFCON call-up, as Fiorentina midfielder Sofyan Amrabat - who is yet to make his Red Devils debut - will be expected to represent Morocco.
Tunisia's Hannibal Mejbri and Ivory Coast winger Amad Diallo could also miss the same period of domestic games, although the latter will miss the first half of the 2023-24 campaign with a knee injury.