Long-tailed weasel. Depending on the injury, the best course might be just to leave the animal alone. If you do decide to seek medical attention for it, be aware that despite the small size, they are typically quite aggressive, so be very careful when trying to handle the animal. Here's a list of the wildlife rehabilitators in Lake County.
Barnswallow / Linda S. Breuer (847) 487-3606
Best Friends Animal Hospital (847) 546-2626
Carla Brown (847) 215-6434 Cynthia Golan (847) 863-5062
Flint Creek Wildlife Rehabilitation (847) 842-8000 Jennifer Boonstra (847) 912-6328 Kathlene Khadra (773) 447-2995
Michael Nader (847) 356-5045
Timothy J. Harris (847) 362-5954