Well CD he did pick Sami in all the crucial games last season and still do
Alright, you've forced me to put my tactical hat on.
Lets take a closer look here...
Its not really Jose playing him in crucial games, its more about those games involving teams whose playing style is one where Sami's attributes are most useful.
This means teams such as Valencia and most of the Champions League teams, teams that are not going to put 10 men behind the ball.
First let's take a step back. If you notice, Sami tends to spend his time in a much more advanced role than Lass. In fact, in many games, Sami's average position is higher than Ronaldo, Benzema, Di Maria, and Ozil, a good example is vs Lyon in the Santiago. In contrast, Lass spends most of his time in Real's own half. He plays a much more conserved role than Khedira and is very rarely seen near the opposition box. His average position is more withdrawn. On a side note this sort of shows he's more tactically disciplined than given credit for. All this could be verified by pulling ESPN's soccernet heat maps and average position maps.
Having said that, why is Sami's attributes more useful against the more dangerous or daring teams? Well, it all comes down to how Sami's playing style affects the role of Real's most important midfielder, Xabi Alonso. If you notice, when Sami is playing against those teams that are not afraid to attack Madrid and aren't parking the bus, he is able constantly make vertical runs and feed the media-punto, usually Ozil. This allows Alonso to silently carry on his horizontal game with the opposition paying him little attention, which means whenever those teams lose possession in offense, Alonso is already in an advanced position and in the perfect spot to dictate the lethal counter attack. That's why when you look at the goals resulting from these counters you'll nearly always notice that Xabi is in an advanced position usually providing that pre-assist pass.
So why do I feel Lass will be an underrated loss? Because of the other La Liga games vs those smaller teams who like to play with 10 men behind the ball, this is when Lass is more effective. Against those teams, the midfield is usually congested with Real finding less space to work with. In those games Khedira finds it difficult to utilize his vertical runs, in those games Sami's best attributes become of no use and his weaknesses are exploited. In those types of games you need a player who can hold the ball, someone who can run tirelessly and routinely win the ball back. Sami isn't the greatest athlete, hence these are certainly not his strengths. It is in those games that Lass thrives. Lass's roles becomes even more important since his constant hassling allows Alonso some breathing room and prevents him being overworked on defensive duties. Therefore overall, Alonso plays deeper with Lass on the pitch as opposed to when playing with Sami.
Essien was essentially a primarily vertical player in Chelsea. In Madrid, following the above analysis, I believe he'll be utilized more in the Khedira role. Essien is definitely more dangerous making those vertical runs and is often seen near the opposition box, as opposed to camping in his own box. He will be basically a more talented version of Samiin Real's midfield. Which means I can see him being utilized more in those games vs those braver team that leave space in the midfield, in a similar way to how Khedira is utilized now, probably translating into more UCL appearances. That's when Chelsea used to invite teams and play on the counter, Essien was essential for their success. However, I don't see him being effective in games against teams packing the midfield with a 4-5-1 formation, especially with his age and regressing stamina, this is when I feel Lass will be missed. Essien won't be able to effectively replace that tireless job, the dirty work that Lass does, and keep up with the younger legs when trying to win back possession IMO.
To sum it up Essien will provide a slight improvement to Sami's role in Real Madrid's midfield, but won't be able to replace Lass's job against the lesser teams, those games where Real needs to grind one those hard fought 1-0 victories that could sway a title race.
Therefore I go back to my original statement, I believe Lass's loss will affect Madrid more than Essien's gain.
...or all this could be a bunch of bollocks