模拟被测服务器上的Golang Func

I'm not in charge of maintaining the code base under test, but I have this method to try to test, which hits a real server:

func (a *APICoreSt) CallServer(context interface{}, apiType APIType, apiURL string, header map[string]string, jsonBody []byte) CallResultSt {

    var (
        Url     = a.BaseURL + apiURL
        err     error
        res     *http.Response
        resBody json.RawMessage
        hc      = &http.Client{}
        req     = new(http.Request)
    )

    req, err = http.NewRequest(string(apiType), Url, bytes.NewBuffer(jsonBody))
    if err != nil {
        //Use a map instead of errorSt so that it doesn't create a heavy dependency.
        errorSt := map[string]string{
            "Code":    "ez020300007",
            "Message": "The request failed to be created.",
        }
        logger.Instance.LogError(err.Error())
        err, _ := json.Marshal(errorSt)
        return CallResultSt{DetailObject: err, IsSucceeded: false}
    }

    for k, v := range header {
        req.Header.Set(k, v)
    }

    res, err = hc.Do(req)
    if res != nil {
        resBody, err = ioutil.ReadAll(res.Body)
        res.Body = ioutil.NopCloser(bytes.NewBuffer(resBody))
    }
    return CallResultSt{resBody, logger.Instance.CheckAndHandleErr(context, res)}

}

Clearly, since this is a unit test, I don't want to be making this call for real, and the test depends on external factors. Is there any way to inject any fakes/mocks, without touching this func itself (it's literally not my job to touch it)?