Real Madrid plays Real Zaragoza tonight. Real Madrid needs to win the match obviously. Well, to be honest Real Madrid needs to win every match from here till the end of the season.
Barcelona play there match against Celta Vigo a good 2 hours before Real Madrid begin their match again Zaragoza so there would be either extra pressure on Real Madrid if Barcelona win their home game (99 percent chance Barcelona will not only win that game but will actually rout Celta Vigo) or there will be even more pressure on Real Madrid if somehow Barcelona drop points against Celta Vigo as that would present a rare opportunity for Real Madrid to gain some ground on Barcelona.
However there is very little chance of that happening as Messi has just become a dad and he would be highly motivated to make an impact tonight. After all, Messi is at that kind of a level that when he wants to score he scores and when he wants to stay a bit closer to his defense line he makes assists that even Alonso would be proud of.
Since Atletico Madrid have upped their level of play as well I think they deserve a mention too. They are level on points with Barcelona. Atletico Madrid play their away match after the Real Madrid match so they would also have some extra pressure of winning the game. I guess there will come a time Atletico Madrid would start dropping points.
That is basically the fundamental difference between Real Madrid and Barcelona and the rest of the team in La Liga. That does not necessarily mean that La Liga teams are weak apart from Real Madrid and Barcelona, it just means that the La Liga division is very competitive. That is why so called small La Liga teams are so competitive in the Champions League.