Just cuz Chelsea are willing to let him go relatively cheap to get a replacement (who?) doesn't mean it's the right option for RM. AT ALL.
Courtois is the polar opposite to an Alisson-type as a keeper. He's the lanky fucker that is almost unbeatable to any shot above the knee on short distance within the 5-yard box and only exceptional precision masters like Messi or Suarez get stuff past him in those situations, because they're able to aim between his legs or just to the side of his hip/thigh. He's also the right guy to reach those amazing semi-low shots from 20-25m by Messi, he excels in saving those.
But that's about it.
RM leave hectares of space behind CBs from beginning of times with any CB pairing, also because of offensive fullbacks the danger is doubled. Courtois only ever played behind deep blocks staying at the penalty box line with early-Simeone Atleti, Mourinho & Conte Chelsea and even with Belgium. He NEVER had to deal with having to make a super rapid reactionary narrowing of the shooting angle that Navas/Casillas types do so well. Look at 3:10 (best example) or 4:30, 4:40, 4:50 of Keylor saves in the video below:
Courtois is best at semi-long shots at bay and making himself HUGE to wingers coming into the box from the side and dealing with those great shots that are near-impossible to reach for most average size keepers. Best exemplified by 0:42, 3:20, 7:03 below. It's mostly the striking elite that find these weak spots regularly, many shit counters get stopped, but his limitations are obvious. Look at 1:00 and how he is GLUED to the 5-yard box:
RM had similar problems with Diego Lopez who was great when Madrid played quite conservative and hesitant to get their line higher than ~30-yards up the pitch. His best games are like United at Old Trafford where he was superb at making the great saves from corners/range and helping out at crosses/corners. But agility wise the taller keeper becomes so overexposed with Real's backline, had many problems in the followup league campaign.
All in all I don't mind having him for that price, even if he's not the right fit. If it backfires on him, we can always bench him and let Navas back in, even if he makes an occasional blunder.