It doesn't have to be in place of someone. Top teams need strong squads. Equating it to Leinster, game time in shared between Devin Toner, James Ryan, Scott Fardy and Ross Molony. Ulster can share game time between Iain Henderson, Kieran Treadwell, Alan O'Connor and Sam Carter.
Agree that prop is an issue. Eric O'Sullivan and Marty Moore are performing admirably but there's no depth. A new loosehead has to be a priority.
I also agree about a new 10. Billy Burns has improved as the season has progressed but again, there's no depth. They could use someone to challenge him.
If they were to recruit from within Irish rugby, Peter Dooley and Tyler Bleyendaal would be high up the list in my opinion. Neither would break the bank, both are Irish or Ireland qualified and both would improve their 23 at the very least.