I wouldnt be surprised if this comes to pass - but i would think its some way of happening.
What happens at the end of the season is that clubs, especially clubs who need dough like us, go to agents and say listen we are prepared to let " player a" go - see what interest/value is out there if any let people others know we are willing at x price. Cue agent goes to clubs and bandies the player about. Sometimes this comes of, other times it doesnt. sometimes the interest doesnt meet the clubs valuation or come to anything in the end etc (often why we head into the final wee of deadline not knowing whats going to happen). We have a list of players we are willing to let go, some you would agree with others you wouldnt and be surprised at but for various reasons their there. Generally this is where rumours spring from. This is basically what managers refer to when they talk about the "market".
During this process a buying club may say to an agent, we arent interested in player (a) that your hawking, but we are interested in player (b )aines. Initially we may say "no way" or "maybe" for 25 mill, they may say 15 mill and the whole thing stalls or turns in to a game of brinkmanship as the window rumbles on. All kind of things can happen in the in between, one or either club may blink and relent as they get desperate in the window, either for money in or the need to get players in. Sometimes the player gets wind of the interest and lobs in a transfer request ( we like this at Everton for many different reasons) which forces the transfer.
I wouldnt rule out Baines leaving, but i would be surprised if it happened early in the window. I dont think we really want to loose him and i dont think he really wants to go. I think we feel we have other players to push first before we would consider Baines an option. I dont think the fee for Baines would be particularly appealling as i would be astounded if Man U paid over 15 mill for a defender - they dont really have to either tbh.
It may come to pass, but there is a lot to happen before and currently its a big if, for me.
We've lost lids before of equal if not better quality then Baines and coped, we will again "If". Its a cost/benifit exercise.