Didn't Vermijl also say something about van Gaal wanting to take two players per position? Although he did take four keepers...
We've taken three conventional number nines in Rooney, Welbeck and W. Keane, with van Gaal perhaps fluctuating between the use of one forward or two in certain games on tour, so three might be the correct number to take while allowing Kagawa and Mata sufficient game time. Rooney and Welbeck are nailed on to go on tour, so like Brightonian has eluded to, maybe it was simply one of the other with Wilson and W. Keane for the other squad berth and van Gaal merely wants to assess Keane more, while Wilson will continue in the U21's.