I absolutely ABHOR this move.
(1) Where is Kovalchuk going to play? People seem to think on the 3rd line, but do you really want that? Hagelin/Eller/Panik has far from set the world on fire offensively, but they have turned into a solid shutdown line that you can use in the playoffs against elite opposing lines, which helps with matchups, especially on the road. Besides, Kovalchuk is listed as a LW and the player he'd seemingly be replacing, Panik, has been on the right side. Does Kovalchuk play both wings? Heck, maybe he'd be better replacing Leipsic on the 4th line.
(2) I realize people don't make much of +/-, but I think it's a stat that stabilizes only over a very long period of time. Kovalchuk has been a negative +/- player for 14 of his 15 years in the league, and is a career minus almost 150. Some of that may be the teams he has played on, but he certainly doesn't instill a lot of confidence in me as a player who brings a high ratio of benefit to risk.
(3) This has the feel of the offensive equivalent of the Shattenkirk acquisition -- a misfit, almost subtraction by addition. This almost seems like a desperation move, hoping maybe Kovalchuk goes on a hot streak and makes the Caps offense overpowering. Or maybe it's a move to add another Russian teammate, to motivate Kuznetsov perhaps?