Gabby Agbonlahor and Ashley Young have been in outstanding form during the first half of the campaign but made little impression against United after a bright opening 15 minutes.
O'Neill said: "Were they affected by Capello's presence? It never entered my head. Was he here today? But then I'm not 21 and wanting to play for England.
"I am sure Fabio will have plenty of time to assess all the English players between now and the summer before the main business starts for him in September with the World Cup qualifiers."