Quoting: littlejerryseinfeld
they currently have 10.4 million in cap space with 7 roster spots to fill, a little breathing room, but not a ton, especially if they want a capable backup for samsonov.
Vanecek can handle the backup role, he's been an All Star in the AHL two years in a row and only costs 700K. Siegenthaler, Boyd, and Sprong are RFAs who'll be re-signed with minimal raises, say 1.2, 0.9, and 0.9 respectively. McMichael and Fehervary are expected to become full time players, and they each cost less than 1M. Kempny's likely on his way out, which saves 2.5M. Taking all of that into account you have 14 forwards, 5 defensemen, and 2 goalies with about 7.5M in cap space to add the last two defensemen. Cap space isn't an issue for the Caps this season.
Of course, even if it was this still wouldn't make sense. Orlov is their #2 defenseman and defense is their biggest area of need, so all of the savings from trading him would have to go right back into finding someone to replace him.