Nagelsmann, whose sacking in March 2023 led to Tuchel's arrival, was tipped to return to Bayern, but has now extended his contract with Germany until 2026
The club refused to comment when contacted by Reuters.
Ronaldo, who missed matches in January and March with hip flexor trouble, is United's top scorer this season with 24 goals in all competitions.
United are sixth in the league on 58 points, two points above seventh-placed West Ham United, and they need a win at Selhurst Park to be certain of securing a place in next season's Europa League.
Sunday's game will be Ralf Rangnick's last as the interim manager, with Erik ten Hag set to be the permanent manager from next season.