Ergo question

After stalking Craigslist for weeks I finally found a good deal on an Ergo. I'm picking it up later today. There is no infant insert with it. The Ergo website says the insert should be used for a baby up to 12 lbs. or 4 months. My LO was 11 1/2 lbs last week at her 2 month check up. Will I need the insert? Is there a way around it, such as using a rolled up blanket? I know I've read of that with some carriers but can't remember if it was the Ergo.