At this stage, it all depends on Atletico Madrid. They need to consistently play the remaining matches and not lose the game to Barcelona. In addition, there is a chance that Barcelona will not be able to beat Real Madrid. It would be better for Atletico Madrid to have Barcelona and Real Madrid draw.
I strongly doubt Atletico Madrid will be consistent for the remainder of the season, they've still got tough fixtures ahead including one against Barcelona, However, be eliminated from all other competitions might help the concentrate and prepare better.
I think Barcelona has a great opportunity to overtake Atletico, as long as they can get the full points against Real Madrid and Atletico. Outside of La Liga, Barcelona only has 1 Copa del Rey final left, so when the Copa del Rey is over, they can really put all their efforts into La Liga.
Thats not the only reason...i believed many players will make the impossible to save the season and maybe prevent messi from leaving the club...also the player himself surly dont want to leave the club in this situation and he want at least to get a trophy to leave in good atmosphere...however i believe messi will never leave Barcelona whatever the cost