The Spain international has been a key figure for the Blues this season, with his 18-goal return helping to guide them to the top of the table.
He has, however, suffered a dip in form since becoming embroiled in a distracting transfer saga during the winter transfer window and is on a five-game drought at club level, having scored just four times in 2017.
Alonso, though, is adamant that the 28-year-old remains a talismanic figure, telling the Evening Standard: “Diego has been massive for Chelsea this season
“He has almost scored 20 [current tally is 18] goals and that is a significant contribution.
“But it’s not just about that, with the way we play, he is very important.
“He had another couple of chances to score against Bournemouth but no doubt next week, he’ll be there again.
“Is he a big-game player? Yes. Manchester United is going to be a tough game for us but it doesn’t matter who scores, hopefully we will get the three points.
“We have a week to rest a little bit and then prepare, that’s what we have to be focused on these days.”
Chelsea will head to Manchester sat seven points clear of the chasing pack in the Premier League title race.
They will also make the journey having already secured a league and cup double over former boss Jose Mourinho at Stamford Bridge this season.