The phrase Alain Vigneault used superstitiously was “touch wood.” “We’ve been — touch wood — fairly healthy up front,” the Rangers coach said Wednesday morning in Ottawa, “other than Mika in the middle.” No, “Mika in the Middle” is not the name of a new sitcom, but there is drama as the Rangers deal with the absence of top-line center Mika Zibanejad, whose delayed onset of concussion symptoms forced him to miss a seventh straight game in Wednesday’s 3-2 loss to the Senators. Vigneault cancelled a scheduled practice for Thursday in New York and Zibanejad skated on his own, something he has been doing for about a week. Yet without a full-team practice, he is unlikely to be ready for Friday’s match at the Garden against the Kings. The same can be said for the second leg of the back-to-back, Saturday’s 5 p.m. start in Boston. With two straight losses(Monday’s a 2-1 shootout loss to the Stars), the Rangers are trying to nip this thing in the bud. Yes, the Rangers have been relatively healthy up front. But as Zibanejad’s injury continues to keep him out, it has become crystal clear just what a big factor he is — and especially to their biggest offensive weapon, their power play.