Large gashes must be closed, either by stitching or gluing. Either requires fine motor skills and the ability to manipulate small items (ex: needle and thread). How could they do that?
First and most obvious to me is the matter of SCALE (no pun intended).
The larger something is, the less fine their work is going to be, and the larger their tools. When a dragon, which is a very large creature, sews up a wound, the spaces in between the stitches would be wider than, say, a mouse. Not that a mouse could do its own stitches or have another mouse do them, but, if they could they would certainly be finer than if a human did them. Depending on the size difference, a curved suture needle might well be larger and more sturdy to get through dragon skin.
So while your dragons might sew wounds shut, it's highly probable they aren't going to be using human sized needles. In this case "small" has a relative value.
Second, I don't know if you have ever seen a suture kit, but, they don't use traditional sewing needles (suture needles are actually curved not straight) nor do they use the same techniques. Here's a practice picture.
You are currently talking about "dragons that can sew" as though the grasping skills and coordination for sewing wounds shut is the same as sewing a piece of clothing. But it is not. While it can be done other ways, in standard practice, both hands are involved. I can sew a piece of fabric one-handed. But sewing leather or a person back together is far more difficult and involves curved needles.
What this means is that opposable thumbs are not enough. A design like a t-rex for example, even with thumbs, would be right out. The hands need to work together, and the vision needs to be good. And with some fantasy dragons, you will see the forelimbs pretty far apart, awkwardly positioned or too short to do what's needed and the dragon still be able to see. Depending on how you design them, it's possible two dragons will be needed to stitch a wound, if they are designed in such a way that hands working in concert and close enough to the eyes are an issue.
That's another thing, this is close work, and for something engineered like this, it's more likely that dragons would be farsighted for combat. While they may be able to do this by feel, I am going to suggest that dragon doctors or stitchers, would be the ones with a little less visual acuity on the battlefield.