Luca Modric situation now is similar to that of Sergio Busquets, some are confused between the difference between their levels with the club and the team, frankly this is not confusing and the reasons are very clear, with the national team you play two to three matches after every two or three months, it is normal that the preparation of individuals here especially Experience elements are better than preparing them for matches after every three days. With the national team, you will not notice the effect of the “age” factor as much as you notice it throughout the week with the club.
There is also another important factor, which is the difference in levels, players like Sergio and Luca in order to benefit from them more throughout the season. You have to rest them in a lot of games so that they don't look tired...