Mattress stitch for ribbing?

I’ve made a mistake with the edges on the sleeves of a ribbed pattern sweater. Instead of having both sides be knit stitches so I can use mattress stich to seam them together, one side has knit stitches and the other side is purled. Is there any way to use mattress stitch for the edges of the sleeves? If not, does anyone recommend any other finish for the seam? Thanks.

You can still use mattress stitch. If you gently pull the stitches apart you should still be able to find the same little ‘bars’ that you pick up to do mattress stitch.