All teams have injuries though, so that's not really an excuse. It is though compounded by your lack of squad depth. As for investment, can you really see any significant investment in the squad when you have a £1bn stadium to pay for ? Levy hasn't ever really invested much before, even when Poch was begging him too, so I don't see why he would now, especially now given your huge debt.
The only way I can see Spurs spending significant sums on players in the market is if you sell your most valuable assets first. Players like Eriksen, Alli, Alderweireld, Kane and possibly Son would fetch good money. As I've said before, you are in a position now where we were over a decade ago when we moved to the Emirates where you have to balance the books and invest in youth and potential, all the time whilst maintaining top 4 and bringing in the CL money.
If you don't, then I think what looks like the beginning of a decline, could accelerate.