United picked up a 1-1 draw at the Bernabeu in the first leg, which means that Madrid must score in the return leg if they are to stand any chance of going through.
While Madrid have picked up consecutive wins against Barcelona in their last two games, Alonso believes that United's recent form makes the match too close to call.
"We've enjoyed two day's decent rest (since defeating Barca 2-1 on Saturday) but both teams go into the game in decent form," he said.
"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."
The match will also mark Cristiano Ronaldo's first visit to Old Trafford since leaving for £80m in 2009.