United scored more goals without Ronaldo than they did after he came in. So the question shouldn't be where do the goals come from if he's gone but instead where did they go once he arrived? This season was the fewest goals the team has scored in the league for years. In the prior season there were three players who scored fifteen or more goals overall in the season. Last season that dropped to one. Channeling so much through him, which is what you have to do when you become a Ronaldo team, seems to be more of an overall negative than a positive for the team at large.
Ronaldo's game is no longer what it used to be but the one thing he's meant to be there to do is give you goals. But if your overall goal tally goes down when you bring him in then really what is he bringing to the team? Tactically what you should want from him s for him to stay in and around the box to make the most of his finishing and short-distance movement. Yet his lack of discipline and insistence on trying to take control of things means he's always dropping deep to collect the ball. So he's then trying to work in the same space as someone like Bruno and also no longer has the burst of pace he used to in order to get back up the pitch to work the ball back into the box.
If Ronaldo does leave then yeh I think you definitely want to bring in a striker to replace him. They already should do with Cavani gone and Greenwood presumably done too. But obviously it becomes more of a priority if Ronaldo is gone.
The link I had posted in that message. You can see
20/21 here and
21/22 here. 20/21 United had an xPTS of 65.64 in the league and in 21/22 that drops to 54.52. The xG (since goals is what Ronaldo is meant to be bringing) for the league goes from 63.17 to 57.21.