Fishel tore the anterior cruciate ligament in her right knee during training on Monday, which opened the door for the veteran forward to be in the squad when the US open their tournament on Wednesday at 04:15 CET against the Dominican Republic.
"As we start this tournament, we're fortunate to be able to add a player like Alex to the roster," said US interim head coach Twila Kilgore.
"Her accomplishments speak for themselves, she's been training extremely hard with the (NWSL's San Diego) Wave in preseason, and I know she'll be more than ready to contribute in this tournament."
When Morgan was left off the initial roster announced two weeks ago, it raised eyebrows and questions as to whether she would be with the team for this year's Paris Olympics.
Morgan, who has a reputation for producing big goals when the stakes are at their highest, has been a key member of US teams that won two World Cups (2015, 2019) and an Olympic gold medal in 2012.
Morgan has won 215 caps, scoring 121 career international goals, placing her fifth on the US women's all-time scorers list.
Following their tournament opener, the US women's team will face Argentina on Friday before closing out the group stage against Mexico on Monday.
The top two finishers in each of the three CONCACAF W Gold Cup groups along with the two best third-placed teams will qualify for the knockout stage.
