The two-time Olympic 100m and 200m champion tweaked the hamstring during Saturday's Cayman Invitational, Mills revealed during an interview on a Jamaican radio channel on Monday, reports CMC.
"If you noticed in the race he was grimacing towards the end," Mills said on Tuesday.
However, Mills said Bolt's injury is unlikely to stop him from racing in Friday's meeting in Ostrava, the Czech Republic.
Bolt is being treated by Hans-Wilhelm Muller-Wohlfahrt, the Munich-based doctor who has attended to him for years.
He raced to an ordinary 10.05 sec in the 100m, defeating American Dentarius Locke and training partner Kemar Bailey-Cole who placed second and third respectively.
Bolt is targeting his third straight victories in the 100m, 200m and 4x100m at August's Olympics in Brazil.