On three different occasions over the past few weeks, The Post has encountered people inside the NHL who either implied or inquired about the possibility of the Rangers going after superlative defenseman Erik Karlsson, who it seems inevitable will be traded by the Senators before the June 22 draft. As appealing as the idea may be of his fellow Swede, Henrik Lundqvist, in net watching as Karlsson’s long hair flies behind him on a rush up the Garden ice, so many questions of practicality arise. Such as, how would his contract extension, which would start in 2019-20 (and would be part and parcel of any deal in which the Blueshirts give up significant assets), fit into the Rangers’ rebuilding plan? (Answer: It wouldn’t.) Even if not every one of the prospects and draft picks general manager Jeff Gorton has assembled since the February Fire Sale pans out to be a big-time NHL contributor, how much upside is there in trading prime assets and limiting the possibility of having a few good young players that bring more financial control? (Answer: Not much.) Once the Rangers traded captain Ryan McDonagh to the Lightning, it was only natural eyes would look to his Ottawa counterpart as a possible replacement. Certainly, Gorton has gotten out from underneath a difficult place regarding the salary cap, which is projected to rise from this year’s $75 million to between $78 million-$82 million next season. It could get even more advantageous if Gorton finds a suitor for Mats Zuccarello this summer and the one year left on his deal at $4.5 million. That would also free up some money for the important group of Rangers restricted free agents, all with arbitration rights — which includes Kevin Hayes, Brady Skjei, Ryan Spooner, Vladislav Namestnikov, Jimmy Vesey and John Gilmour, all of whom suited up for another audition against the Blues on Saturday night.