Word's grammar check tells me the second sentence below requires a semicolon rather than a comma
“If you don’t accept your physical limitations, you will burn out.” Elena was analytical again, the empathy absent from her tone.
This accepted answer tells me the semi-colon is probably OK (contrary to multiple other sources cited in comments to the question), but is probably a sign that my prose needs to be reworked.
It could be lengthened to
Elena was analytical again, and the empathy was absent from her tone.
Is my original version incorrect (why)? If I use a semi-colon as Word suggests, it would be the only one in the novel. What other options do I have to keep the sentence crisp without altering the meaning?