"They are still in with a great shout.
"I think that United are the only side capable of winning 14 or 15 games in a row if they do not have any disruptions. You certainly cannot write them off for the Championship.
"They are on a roll now and they will be a threat for the title," Robson said after the game at The Hawthorns.
Robson pointed to the loss through injury of Darren Moore and Cosmin Contra as the reason the Baggies succumbed to three second half United goals.
"We had to reshuffle the side and had players playing out of position. It unbalanced us and made it a difficult day all round.
"We went to sleep for United's first goal and once they go in front you know that it is difficult to get the ball back off them.
"I am disappointed and we have now got to work hard for the rest of the season to put things right."