Mulumbu faces a three-match ban and potential club fine for foolishly picking up the ball and kicking it at United's Gary O'Neil at the end of the 3-1 loss at Upton Park.
The Congo international took to Twitter after the game and apologised to the fans, admitting: "I'm really sorry about what happened.
"It doesn't look like me. Not a good example for the kids and all WBA supporters."
In response, Foster said: "Don't you worry about it mate, you're Mr Professional, everyone slips up some times."
Boss Steve Clarke has, however, stated Mulumbu's absence for the next three games will be "a big loss" for the Baggies.
- Sports & Recreation