The Red Devils waved goodbye to record clean-sheet holder David de Gea after 12 years of service this summer, as the Spaniard exited the club upon the expiration of his contract.
Despite winning the 2022-23 Premier League Golden Glove, De Gea's final season at Man United was marred by a handful of errors, and no agreement could be struck over a new deal with a significant pay decrease.
With Dean Henderson also being allowed to join Crystal Palace on a permanent deal, leaving Tom Heaton as the club's most senior stopper, the Red Devils' recruitment team sought moves for a pair of goalkeepers ahead of the 2023-24 season.
A multitude of names were linked with transfers to Old Trafford, including Everton's Jordan Pickford and Villa's Martinez, before Man United settled on Andre Onana and Altay Bayindir from Inter Milan and Fenerbahce respectively.
According to The Independent - via the Manchester Evening News - Man United chiefs were eyeing a move for Martinez, who had won the Golden Glove with Argentina en route to World Cup 2022 glory.
However, Ten Hag blocked the club from making an official approach for the former Arsenal man in favour of a reunion with Onana, whom he had coached during their successful stint at Ajax.
The Dutchman's desire to bring Onana to Old Trafford ultimately won out, but the Cameroonian stopper has endured a turbulent start to life at Old Trafford, conceding 30 goals in his opening 18 games.
Onana has often been the subject of criticism for perceived poor displays, and he has already shipped 11 goals in four Champions League contests, including four to Copenhagen in midweek.
The 27-year-old has not been without his positive moments at the Theatre of Dreams, though, saving a last-minute penalty from Jordan Larsson in a 1-0 victory over the Danish side two weeks before their 4-3 loss in Group A.
Onana kept his sixth clean sheet of the season in all competitions in Saturday's 1-0 win over Luton Town, which was his second league shut-out on the bounce after last weekend's identical triumph at Fulham.
Meanwhile, Martinez has four shut-outs to show from his opening 15 games of the new term, only registering two clean sheets in the Premier League and conceding in each of his last five top-flight games.
Nevertheless, the 31-year-old won the Yashin Trophy at this year's Ballon d'Or awards on account of his World Cup exploits with Argentina, while Onana placed sixth in the ranking after helping Inter Milan reach the final of the Champions League.
Man United return to action after the international break with a trip to Goodison Park to meet Everton on November 26, before trying to keep their Champions League dream alive against Galatasaray three days later.