Harvey signed a one-year, $11-million deal with the Angels this past winter. The 30-year-old right-hander has authored a 7.09 ERA and 6.35 FIP over 12 starts this season.
With Harvey designated, the Angels have seven days to decide their next move, with a trade or outright release being the most likely options.
"We felt it would increase our chances of winning games," Angels manager Brad Ausmus told Maria Torres of the Los Angeles Times of the decision to remove Harvey from the 40-man roster. "Matt was a very well-liked teammate. A guy everyone pulled for, including myself. But we're in the business of winning baseball games."
Injuries derailed Harvey's career after he earned an All-Star nod with an MLB-best 2.01 FIP over 178 1/3 innings in his second season in 2013; he's since required Tommy John surgery and suffered from thoracic outlet syndrome.
"When you're there on a one-year contract and you have younger guys you want to look at, they're going to make their own decisions," said Harvey's agent, Scott Boras. "So I don't think (this) has a lot to do with Matt Harvey as much as it does what the Angels wanted to do."
After posting a 7.00 ERA over 27 innings with the New York Mets last year, Harvey was designated for assignment and was eventually traded to the Cincinnati Reds in exchange for catcher Devin Mesoraco.