Fellaini insists the Reds must stand up together after falling to a third defeat on the bounce at Watford on Sunday afternoon.
"At half-time, the manager told us to come out and play with confidence and we did that by grabbing the goal," Fellaini told MUTV. "We played with tempo and we need to start doing that from the first minute.
"In the second half, we were much better. We kept the ball, created chances and played forward.
"We got back into the game at 1-1 but then we gave away the second goal by giving them too much space and not pressing the ball.
"We need to focus. At the moment heads are down but, as I said, we must stand together and fight and show that we are confident."