The objective of this study was to compare treatment options for acute management of anterior cruciate ligament (ACL) injuries using preclinical models. Twenty-seven adult purpose-bred research hounds underwent knee surgery (sham control, exposed ACL, or partial-tear ACL) and were assessed over the following 8 weeks. Dogs were randomized into three treatment groups: standard of care (i.e., rest and nonsteroidal anti-inflammatory drugs [NSAIDs]), washout, or leukoreduced platelet-rich plasma (PRP) so that a total of nine dogs received each treatment. Data from the two ACL-injury groups were pooled for each treatment (n = 6 per treatment group) and analyzed for treatment effects. The washout and PRP groups experienced less lameness, pain, and effusion, and greater function and comfortable range of motion compared with the NSAID group, with the PRP group showing most benefits. PRP was associated with the lowest severity of ACL pathology based on arthroscopic assessment. Measurable levels of inflammatory and degradative biomarkers were present in synovial fluid with significant differences noted over time. Based on these findings, washout had positive clinical effects compared with the standard-of-care group especially within the first week of treatment, but became less beneficial over time. A single injection of leukoreduced PRP was associated with favorable clinical results. However, no treatment was significantly “protective” against progression toward osteoarthritis after ACL injury.