"It was a lack of discipline by Nigel and he let himself and his team-mates down. He will be fined," Robson said after the 2-0 defeat.
"He kicked out at George Boateng and the referee was right to send him off.
"But it is not just the fact we lost him in this game we will lose him for four in the future as he was sent-off earlier in the season playing for Southampton.
"You cannot afford to have ill-discipline like that in your squad. We caused Middlesbrough problems with 10 men and who knows what would have happened with 11 still on the pitch.
"I felt that we were by far the better team but we gave away a cheap first goal and you cannot afford to do that in the Premiership.
"Our general play was good but we lacked a cutting edge. Diomansy Kamara and Jonathan Greening had some great chances.
"But, even with 10 men, we showed a lot of character and stuck at it.
"This is obviously a setback for us but we have been in worse positions before this season."