Louis van Gaal was unhappy with a 'stupid' booking leading to Juan Mata's red card in Manchester United's 1-0 loss at West Brom.
The Spaniard was sent off for the first time in his career for picking up two quick yellow cards, the first for preventing a quick free-kick and the second for tripping Darren Fletcher.
Salomon Rondon then scored for the Baggies after the break to end United's winning run and dent their top-four hopes.
Asked about the red card awarded to Mata, Van Gaal told Sky Sports: "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.
"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."
The defeat sees United remain three points behind fourth-placed Manchester City having also played a game more.
"It's a big blow, these kinds of matches you have to win," said Van Gaal. "We have played 60 minutes with 10 men against 11 and this is the fifth match in 14 days.
"It is very difficult to play against 11 and that's the reason we lost. We didn't give many chances away, only the goal really, it's a big pity that we had to play with 10."
With nine games left to play, Van Gaal now believes United need maximum points to finish in the top four.
He added: "We need 27 points, it's a lot but it is always possible."