It appears that it didn't really matter that the main star of last night's "Rent" musical on Fox was injured and they had to air a lot of pre-recorded segments, because the viewing audience was not very high in the first place. According to fast nationals, the special live musical only grabbed about 3.4 million viewers for the entirety of its broadcast, which makes it the least-watched musical so far that has aired in the last 6 years.
TVLine put together a list of all of them, and unfortunately for "Rent", it was at the very bottom.
Sound of Music Live (NBC, Dec. 2013): 18.6 mil/4.6
Grease Live (Fox, Jan. 2016): 12.2 mil/4.3
The Wiz Live (NBC, Dec. 2015): 11.5 mil/3.4
Peter Pan Live (NBC, Dec. 2014): 9.2 mil/2.4
Hairspray Live (NBC, Jan. 2017): 9.1 mil/2.3
Jesus Christ Superstar Live (NBC, April 2018): 9.6 mil/1.7
The Rocky Horror Picture Show (Fox, Oct. 2016): 5 mil/1.7
The Passion (Fox, March 2016): 6.6 mil/1.6
A Christmas Story Live (Fox, Dec. 2017): 4.5 mil/1.5
Rent (Fox, Jan. 2019): 3.4 mil/1.4 (per fast nationals)