A number of MLB players have earned their large salaries as a result of their past success. This does create a situation for a number of players where they do not live up to the expected performance that they should have for what they are being paid.
This does not mean that these players will put up poor numbers. Some of them will have great seasons, but they won't be enough to justify the more than $20 million that some of them are being paid.
As the average salary of MLB players has gotten bigger and bigger, there have been more and more players that have gotten money that they do not exactly prove that they deserve.