Mourinho claimed new Chelsea boss Frank Lampard was wrong to pick inexperienced youngsters Abraham and Mount against a leading Premier League team like United.
The former Chelsea manager said he was surprised proven veterans N'Golo Kante, Olivier Giroud and Marcos Alonso started on the bench as United ran riot at Old Trafford on Sunday.
Lampard expressed surprise at Mourinho's comments and Azpilicueta made it clear he didn't agree with the assessment of two of Chelsea's more highly-rated academy graduates.
"If they are here it is because they deserve to be. They are fighting in every training session. We trust in them," Azpilicueta said.
"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.
"Obviously everyone would like to start in a better way but we have to face the reality and become stronger from it."
Abraham and Mount started brightly at Old Trafford, the former rattling the post, and Chelsea were perhaps unfortunate to be trailing at the break following Marcus Rashford's penalty.
But they collapsed in the second half, making it a miserable first Premier League game in the dug-out for Chelsea legend Lampard.
Azpilicueta has urged Chelsea to draw a line under the Manchester meltdown as they head to Istanbul to face Liverpool in Wednesday's European Super Cup final.
"We know that Chelsea always fights for everything. When you don't win, the questions around get more and more but we have to cope with it," he said.
"We know that playing for Chelsea requires your best effort in every game so we have to learn from the game.
"We know that we can improve. We have to face it, take the responsibility and move forward. We have a trophy to fight for on Wednesday against a good team and we have to be ready for it."