Agreed. Surely the only reason Koeman feels the need to play Klaassen, Sigurdsson and Rooney in the same team is because of the amount of money outlayed on the former two, and because the latter is, well, Rooney. Doesn't make any sense purely in footballing reasons, as all three are played best occupying the same area of the pitch, they are not quick, nor are they frontmen.
They need to be rotated with Lookman, Vlasic or Mirallas played to give us much-needed pace and width. Siggy and Klaassen are Koeman's signings though, so until either 1) there is a big turn around in Klaassen's form, or 2) Koeman swallows his professional pride and realises it ain't working, then we might be in for a long next few weeks.