The Spanish playmaker was handed the captain’s armband at Old Trafford and netted the winner with a free-kick late in the second half.
Mata was handed the responsibility after Wayne Rooney, Chris Smalling and Michael Carrick missed the game.
And the Manchester United boss has revealed that he did not have to think twice about making Mata captain.
Speaking to MUTV, Van Gaal said: “It was not difficult [to make Mata captain] because I have an elder group of players with whom I’m talking about aspects of things – training sessions, meetings or how we have to approach players and so on.
“Juan is a member of that group with the three captains – Rooney, Carrick and Smalling – and [Bastian] Schweinsteiger as well, so it was an easy choice for me.”
Manchester United head to West Bromwich Albion on Sunday afternoon looking to continue their recent improved form.
The Red Devils have won their last four games in all competitions, including wins over Manchester United and Watford, to move them level on points with fourth-placed Manchester City in the Premier League table.
United take on Liverpool in the last 16 of the Europa League on Thursday night.
MORE: The latest football news