Background: Surgical treatment for rectal cancer may cause complications that exacerbate the condition, lengthen hospital stays, and raise healthcare costs. Rectal cancer surgery complications can be detected immediately with postoperative C-reactive protein (CRP) monitoring. Elevated levels of CRP indicate the presence of inflammation and can serve as a predictive factor for future outcomes. Methods: A retrospective cohort analysis was performed on 67 rectal cancer surgery patients. Prior to and after surgery, CRP levels were measured on days 1–3 and 4–7. Postoperative complications were linked to CRP, surgical approach (open, laparoscopic, conversion), and patient characteristics. This study included t-tests, chi-square tests, Mann–Whitney U tests, ANOVA, Pearson and Spearman correlation analyses, and logistic regression. Results: There was a significant correlation between high levels of CRP on postoperative days 4–7 and the development of problems (p < 0.001). The ratios of CRP/albumin and CRP/platelet were highly predictive of problems over this period (p = 0.000033). Patients who encountered problems had a notably greater estimated blood loss (p = 0.0086). Logistic regression analysis demonstrated a statistically significant relationship between higher Charlson Comorbidity Index (CCI) scores and an elevated probability of experiencing problems (p = 0.0078). Moreover, patients who underwent laparoscopic surgery but had to be converted to an open method saw a notably greater incidence of complications (p = 0.0022). From a financial standpoint, the average cost per patient with complications was EUR 1128.75, resulting in a total cost of EUR 44,021.25 for all 39 patients. Conversely, patients who did not experience any difficulties had a cost of EUR 731.25 per patient, resulting in a total of EUR 20,475.00 for all 28 patients. Conclusions: Regularly monitoring CRP, particularly between days 4 and 7 following surgery for rectal cancer, can promptly identify any complications. Monitoring CRP levels and promptly managing any abnormalities can enhance surgical outcomes and reduce healthcare costs.