The Australian international, Boro's leading scorer last term, has scored just once so far in the Championship this season but his manager is keeping the faith.
McDonald had a great chance to break his barren streak of nine league matches when he rounded Southampton keeper Kelvin Davis but was unable to convert in the 3-0 away loss on Saturday and despite that, Mowbray is backing him to soon get amongst the goals.
"All centre-forwards who haven't scored goals for a while have to believe in themselves and keep going," he told the club's official website.
"It would have done him a power of good if that had gone in just before half-time.
"At this moment things are not working but he has to keep going and believe he'll score in the next game."
It might not be a long wait for McDonald as fourth-placed Boro next meet second bottom Doncaster Rovers at the Keepmoat Stadium on Tuesday night.