Are "not because" and "not just because" opposites?

Question

"They aren't playing as well as they always do. Not because they have lost motivation, but because their key players are suspended."

Which of the following is implied in the above example:

1. They have lost motivation, but that has nothing to do with their not playing well.

2. They haven't lost motivation at all, and the only reason they aren't playing well is the absence of their key players.

If number 1 is correct, that means "not just because" is only a more emphatic version of "not because".

But if number 2 is correct, does that mean "not just because" means the exact opposite (i.e. they have lost motivation, and while that partly accounts for why they aren't playing well, the other and perhaps the main reason is the absence of their key players)?

Answer 1

Your second interpretation is nearly correct: the team are hampered by the suspension of their key players and the implication is that they are still motivated. However, even if they are not motivated, the speaker says that the suspension is the cause of the poor performance.

"...not just because they have lost motivation..." doesn't just suggest that the team has lost motivation, it positively states it and also states that the loss of motivation contributes to the loss of performance. However it also states that the suspension contributes to the poor performance.

To turn the situation on its head we can say "They aren't playing as well as they usually do, not because their key players are suspended but because they have lost motivation. In this case we would be dismissing the obvious problem (the suspension) and saying that the cause of the poor performance is a loss of motivation on the part of the team members,