How about a hospitalier?

There are those who are not proper sisters of battle. Who just go about helping people.

Or similar.
Those have great knowledge of medicine. Astartes physiology is sturdy enough to fix itself with some monitoring. And for me.... well, i'm not totally mechanical.
So i'll just make sure not to get any important bits blown off. I should be fine.

Also, in a pinch, (and if he would want to), the techmarine can fix up a techpriest without much problem.
Most of the systems are exactly the same.
So with only a few "Wait, where does this part go? Oh, right." It would go fine.

But yeah true, i guess i sorta forgot Oskar a bit. However he is a criminal and looks the part....

Nah, it will be fine. I'm sure you can come up with a character. :)
I myself never try to fill in skill gaps in the group. I make my character. Then the group might have severe problems. But one can solve that in game!