Some say this signing would be very short-term, but it's in the short-term that United needs to act to gain in the long term. Having already missed CL qualification twice in the last 3 years, it's crucial to get back into that top 4, and CL qualification, in order to salvage what's left of that UEFA coefficient.
Right now, United is 20th, behind the likes of Basel and Leverkusen. If United wants to challenge for top honours again in the next few years (including CL), you don't want to end up stuck in a CL group with Real/Juve/Bayern/Barca, running the risk of ending 2nd and facing one of those teams yet again, before the finals have even properly started.
Winning, or challenging for, the Champions League may seem a little far fetched atm, but for a club of United's stature, this should be the eventual goal and getting Ibra in for next season can only help in reaching that long term goal.