If he doesn't have any major injuries how does he lose enough blood to need a transfusion?
If it's a slash, does it open up the abdominal cavity or just go through surface skin and muscle? A slash that goes into the abdominal cavity is likely to sever a bunch or organs, vs. a puncturing stab which may miss organs.
What kind of recovery time do you need? Depending how deep/long the cut is, your character might need to avoid heavy lifting and activity for a few weeks. As someone who has had minor abdominal surgery I can say that it feels terrible to have an injury to your stomach area, and that was just tiny punctures so they could look around with a camera. Your body just wants to protect it, and it's hard not to walk all hunched over even after it's healed. You can tailor your character's injury to what you need for the story though.