Manager Frank Lampard has been unable to add to his squad this summer as Chelsea are serving a FIFA-imposed transfer ban.
In his first Premier League game in charge, the Blues legend picked Mount, 20, and Abraham, 21, in his front four.
Lampard's decision to put his faith in both, the products of Chelsea's youth academy, was criticised by some, including former Chelsea manager Jose Mourinho after they were thrashed by United in their 2019-20 Premier League opener, per Sky Sports.
But Azpilicueta said all the players in the squad deserved to be there, per James Olley in the Evening Standard:
"We trust in them. We know that we have a fantastic academy and they have had fantastic experience on loan. If they are here it is because they deserve to be. They are fighting in every training session. All the boys, they train to play for the Chelsea first team since they were young and now we have a massive opportunity to create a fantastic group, to learn from every game. Yesterday, obviously everyone would like to start in a better way, but we have to face the reality and become stronger from it."
Mount played in the Championship under Lampard at Derby County last season, and the midfielder was a key player for the Rams as they made it to the play-off final.
Abraham, meanwhile, was also playing in England's second tier in 2018-19 with Aston Villa.
The forward netted 26 goals in 40 appearances as Villa were promoted to the Premier League by beating Derby in the play-off final:
Despite both flourishing in the Championship last term, it was always going to be difficult for the pair to make an immediate impact in the Premier League, especially given Chelsea's opening game was at Old Trafford.
There were still encouraging signs for Lampard in Manchester despite the heavy defeat:
And the Englishman also has key players to return like Antonio Rudiger, Willian and N'Golo Kante.
He has a decent squad, though, and Chelsea's youngsters are likely to be given more opportunities than ever under Lampard as he cuts his teeth in Premier League management.