Manchester United v Manchester City - Premier League Preview
The United boss has stuck with the England international since Rojo was ruled out with injury.
But sickness last time out cast doubt over Smalling's inclusion in the key derby clash with Manchester City today (4.00pm).
However, Van Gaal opted for consistency with Smalling at the back , insisting it's best to enter difficult games 'as a team'.
He told BBC Sport: "It's always a tough decision because all the players are doing their upmost.
GETTY
Louis van Gaal says a team mentality is key going into big games like the derby
Its a very important game and Marcos wanted to play also
Louis van Gaal
"Its a very important game and Marcos wanted to play also.
"You start the match better I believe as a team, but a derby is always special and there are a lot of factors you cannot control as a manager, player or team.
"You never know how the game develops, you have to wait and see."
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