See, if we signed either of those players I wouldn't automatically feel yes everything is fine now, most likely it's my fault for not recognising the importance of that role in a team.
You're certainly not alone there and in fact I think that I'm in the minority when it comes to believing in the importance of an excellent holding midfielder - perhaps because it's the position that I played, but that's beside the point.
Regardless, I feel that we'd see a significant improvement with a good DM (note the win % difference with Lucas in the team - it's typically something like 1.5x - 2x). And that's not because Lucas is the best player we have, it's because he fills a role that nobody else can and as an externality to that, he improves the side vastly.
The objective of that role is to serve as a guardian to the back four in not only making tackles, but intercepting key passes, cleverly positioning yourself to force offenses in certain directions, winning aerial battles to break up play and take possession, and bring structure and guidance to the back four and midfield as a unit.
In terms of other duties, the DM should also be capable of finding passes that can remove the ball from pressure, launching attacks, consistently be an option for any player on the pitch to pass to, recycle possession, and once again serve as the connection of ball movement between the back four and our midfield/attack.
Outside of Lucas, there is nobody in our side that can do this and I think we'd be doing our defense and our "goals against" statistic a massive favor by either acquiring someone better who Brendan has more faith in that can fill the role, playing Lucas there, or grooming Emre Can to play that position.
Utilizing that role will do a lot more than have a simple, direct impact, but it also serves as a huge impact in terms of team structure, opposition team's offensive abilities and movements, and our possession abilities. The fact that Brendan hasn't realized this yet is mind-boggling to me personally.
Again these are all just facets of my opinion and may not be the case whatsoever, but I like to think that a lifetime of dedication to playing, watching, and studying the sport gives me at least some foundation of knowledge to understand what makes a team tick, but a lot of people out there have the same, if not more experience/knowledge than myself and may very well say differently!