The move is significant for Manchester United because it opens the door for Marouane Fellaini, who he will likely be replacing, to leave the club, with United heavily linked to his signing all summer.
There are no reports of any formal bids today just yet, but that could change in the coming hours as United chase the signings of several players. Fellaini had a release clause earlier in the season, of course, but in our infinite wisdom, we decided to wait until it expired before bidding to get him at a lower price. That almost certainly won't happen, which is quite staggeringly incompetent when you think about it (something we don't advise doing.)
Anyway, with McCarthy off to Everton, and Everton extremely and terminally broke, it seems that they'll be making one big sale today regardless, and Fellaini to United is the overwhelming contender.