Romanian Halep was scheduled to play Veronika Kudermetova in the second round in Cincinnati on Wednesday after overcoming Anastasia Potapova on Tuesday.
She pulled out before the clash, though, giving Kudermetova a walkover into the last 16 where she will face either Paula Badosa or Ajla Tomljanovic.
World number six Halep won her biggest title in two years last week at the Canadian Open.
A 6-3 2-6 6-3 over Beatriz Haddad Maia saw her move back into the world's top 10 for the first time in a year.
With the US Open starting in less than a fortnight, Halep will hope the injury is not too serious.