As others have said, when a new carrier comes onto a route already served by 2 airlines, it's the extent to which it stimulates growth as opposed to re-distributing existing business, as well as maintaining yields, which are important.
Lisbon is interesting in that as far as I know easyjet are transferring the route from LPL where it did pretty well for numbers based on posts on the LPL thread. Whether those customers will be happy to fly that particular route from MAN or for convenience choose an alternative destination remains to be seen.