Assuming for a moment that all this is true, and United and Mourinho have already done/signed an agreement or whatever, then who would be the likely source be for leaking this information to El Confidential, COPE, or whomever else at this stage now?
If it were from Mendes, then surely he would be briefing the opposite to try to put pressure on United for his client's benefit? What does he gain from the press reporting a done deal if the deal actually is done? Now if there were no deal - or maybe even no interest from United at all, as I think is the case - then I could understand leaking a bullshit story linking his client with United in the hope of flushing-out a PSG, or Valencia, or Hong Kong Pandas (I, erm... don't actually know the name of any Chinese clubs); we see this tactic in action every summer in the transfer market so why not with unemployed managers during the season?
Alternatively, why would United decide to leak this development if it has actually happened? The club wouldn't want the press (even the foreign press) reporting this news in February when Champions League qualification is very much in danger - why risk destabilising the club at this stage? I don't immediately see how United would benefit from leaking the news, so I'd rule the club out as being the source.
Other possible sources could be a rogue employee from either Mendes or United, or a third-party source involved in the deal (legal advisers maybe?), or even just a random chancer who has somehow found-out the details and decided to cash-in, rather than the leak being authorised by either Mendes or United as a discussion tactic. Or maybe the source comes from LVG, stemming from being upset at not getting his anticipated third year?
Or it's just completely made-up, in order to generate interest/cash during the post-transfer market lull for the media, or just to misled the media altogether.
In summary: I'm sceptical, but we shall see I guess.