[Lance is just going to raise an eyebrow here, clearly not all that amused by this "bad arm" excuse.]

Aw c'mon, Shiro! Don't be a stick in the mud!

[Ha. Stick. He's hilarious.]

You just got out of stasis, man. You've gotta work off that old sleep chamber arm somehow!

[He's sounding enthused as always, but beneath it all there's some concern, too. Last he saw Shiro he had that familiar Galra roboarm and this one is clearly pretty different and as much as he wants to ask bluntly how the heck Shiro got this one or why the other arm really needed replacing in the first place, he can't for the life of him think of a tactful way of doing it that won't also bring down the mood. Instead:]

Besides! I wanna see how cool your new arm is!