You're really talking about Rom AND Kone getting injured in the last 15-16 matches of the season, AND the results making a material difference on our final position in the table. As a gamble, those odds are very, very low.
Apart from maybe the early rounds of the FA Cup, Rom plays every minute of every game for us, no matter how many games we've got. It would be frankly amazing if he doesn't get an injury at some point (praying that he won't). So are you happy with Kone as the backup, because it will only take one twinge or one accident in training and that's where we'll be?
Say Rom is out for 2 months at the beginning of February for example. Are you happy for us to go and play the rest of the season (including away at Anfield, a possible cup final etc) with Kone?
I'm with pjk. The manager's job is to ask 'what if?' and to cover all possible eventualities within reason. If he doesn't go and get us another striker in the next week or so, then he's basically playing roulette with what's left of our season. It would be an appalling piece of decision making to just hope that Rom makes it through. And loads of people are pointing it out on here, so it's not like other people can't see it.