The Spanish midfielder was booked by referee Mike Dean for preventing a free-kick in the 23rd minute before being given his marching orders when catching Darren Fletcher just three minutes later.
United went on to lose at The Hawthorns due to Salomon Rondon's lovely control and finish on 66 minutes and van Gaal suggests perhaps it was due to Mata's ill-discipline although he did hint that perhaps the second yellow was a little harsh because of the nature of the player.
"The first was a stupid yellow card, you know it is a yellow card in advance. The second you know you already have one so you have to take that into account," van Gaal told Sky Sports.
"You damage the team, but the referee has to know the person who makes the tackle and Mata never hurts an opponent.
"The referee has to know the character of the players and that is why an experienced referee is better because he knows the players and the game."