Disclosed herein are systems and methods for treating a patient that has undergone a bariatric surgical procedure, to promote weight loss in the patient. The systems and methods can involve positioning a gastrointestinal bypass sleeve within a portion of the altered GI anatomy to create or restore a restriction, and/or create a malabsorptive effect via a gastric and partial intestinal bypass. The bypass sleeve can include a proximal attachment element for attaching the proximal end of the sleeve in a penetrating or non-penetrating manner at the gastroesophageal junction, stomach, neo-stomach, or other locations. The bypass can be placed during the same operative session as the bariatric surgical procedure, or alternatively at a later date.