There will be no major repricussions for MAN at this stage.
MAN-LHR is now at a sustainable level to remain competative. Dont forget, there are a lot of passengers going east, and there are far more direct options from LHR going west too, DUB will never be a full alternative to LHR, unless EI have plans to match BA's frequency to the USA?
Also, aside from the winter months when ORD has to be rested for 2-3 months, MAN has traditionally been a strong market for AA. There is a reason they have been here since 1985. And its also worth pointing out MAN is now an all widebody station for AA, which again translates into the fact the capacity must be needed at MAN, after all, if a B757 can do the job then why bother upping to the B767.
The EI deal doesn't suddenly mean that 2 B767's, an A330 and potentially 2-3 airbus worth of pax per day suddenly disappearing to DUB. At (supposedly) 6 daily and another 6 daily from FR I doubt any more frequency will be added as the main hub banks would be covered given the main bulk of TATL goes in the morning, and just how many more people do they envisage being shoved into an A320 to meet that morning wave?