The temperature at BBVA Compass Stadium in Houston was 92 degrees with a heat index of 102 when the game kicked off at 3 p.m.
Players from both teams appeared very exhausted at the end of the match, and Daly - who appeared to be struggling on the pitch and staggering at times in the second half of the match - eventually collapsed on the pitch just as the final whistle was blown.
The 25-year-old England international was lightly jogging after taking a shot on goal before going straight down to the ground and rolling over.
Her teammates immediately went over to her and called the paramedics. She was then stretchered off the field and taken to the hospital, where she was treated for heat illness.
A little after an hour after the game, the NWSL released a statement via twitter.
Daly, who has now been released from the hospital and is resting at home, then posted a message on Twitter thanking fans who wished her well and her teammates and staff for their support.
NWSL Statement on Houston Dash vs. Seattle Reign FC Match: pic.twitter.com/sM2Rfdt6k5— NWSL (@NWSL) May 27, 2017
Thank you to everyone pic.twitter.com/nWDbt4qRIP— Rachel Daly (@RachelDaly3) May 28, 2017
Pure class from everyone involved. Cannot thank these people enough. https://t.co/QdAkwY4Ue5— Rachel Daly (@RachelDaly3) May 28, 2017