The Spaniards head to Old Trafford having snared a 1-1 draw after falling behind in Madrid last month and the midfielder is aware that it will be a difficult clash despite the fact Real's domestic form has been brilliant recently.
"We've enjoyed two day's decent rest (since defeating Barcelona 2-1 on Saturday) but both teams go into the game in decent form," said Xabi.
"I think independent of what has happened in the last week in the two different competitions (Copa del Rey and league) we are coming into the game in good form and so are Manchester United.
"It is a very open tie, we come here knowing we need to score and they know they have to defend properly but it will be very tight and close to call.
"It is a massive game and will be decisive in both teams' seasons."