The opposite would be as expected, or unsurprisingly.
The opposite would be believably or credibly. However, the adverb is often used to mean unexpectedly, where the opposite would be obviously or as expected.
Zemblanity, the opposite of Serendipity. Serendipity is the unexpected happening of something with pleasing outcome.
Unexpectedly is an adverb.
He had no illness, so Harvey's death came unexpectedly. Unexpectedly, the couch exploded. It was an unexpectedly brief encounter. Unexpectedly, the monkey counted the beans correctly.
He came to my house unexpectedly
He had arrived unexpectedly and they were delighted to see him.
The root word for unexpectedly is "expect."
Surprise would be the world. It is unexpectedly sudden.
Unexpectedly, an adverb, means surprisingly. Some synonyms of the term "unexpectedly" include out of the blue, quickly, startlingly, suddenly, and unusually.
I was very happy when my brother came to visit unexpectedly this weekend.
The star of Bar Rescue television series decided to drop in unexpectedly on the bar he helped.We wanted to drop in unexpectedly on my sister as a surprise. The Child Protective worker hoped to drop in unexpectedly on the foster home to check on the care.
He took a week off when his mother died unexpectedly last year.