Barca have a six-point advantage over the champions with seven matches left and can move a step closer to a first Spanish title in three years with victory over third-placed Sevilla at the Nou Camp on Wednesday.
Real have a chance to cut the Catalans' lead in half when they host city rivals Getafe on Tuesday and know that the clash against Barca on May 3 could be a last chance to prevent Pep Guardiola's side taking their league crown.
Diarra, who is suspended for the Getafe match, said it was perfectly possible Barca would drop points against Sevilla and at in-form Valencia, in fourth, on Saturday before they come to Madrid the following weekend.
"Barcelona have two very hard matches and then the Champions League (semi-final against Chelsea) but the title will be decided in the Real Madrid-Barcelona match," Diarra said in an interview published in Monday's Marca newspaper.
"It very much has that look about it and it's going to be a spectacular game," the French international added.
Under coach Juande Ramos, Real have won 15 of their 16 league matches since losing 2-0 at the Nou Camp in mid December, staying in the hunt for a third consecutive title and hoping Barca's packed schedule would cause them to slip up.
PACKED CALENDAR
Barca are still alive in three competitions - Spanish league, King's Cup and Champions League - and midfielder Xavi said he was feeling neither physically nor mentally tired.
"It's a privilege to have a calendar as packed as ours and to have a chance to compete for glory," the Spain international told the club's website.
"They (Real) are talking about something hypothetical whereas I talk about realities," he added. "We are the ones who are in front and we are the ones with the six-point lead."
Barca's task against Sevilla will be made easier by the absence of the Andalusians' first-choice goalkeeper Andres Palop and defender Adriano Correia, who are suspended.
Valencia play at improving Real Betis on Wednesday on a run of four straight league wins that has lifted Unai Emery's side above Atletico Madrid and Villarreal into the fourth Champions League qualification spot.
On Thursday, Villarreal host struggling Recreativo Huelva and Atletico travel to Santander to take on Racing.
Recre are in 18th on 30 points and Espanyol and Numancia fill the remaining two relegation places on 29 and 28 points respectively.
