Susceptible

I think it means he is more likely to receive valid criticism.

Looking at a slightly different example:

"He is susceptible to flattery."

Does it mean he is easily fooled by fake praises, or he is more likely to be praised than others?

He is easily fooled.

So "X is susceptible to Y" could mean either:

1. The probability of Y happening to X is now higher.
2. Y now has a greater affect on X.

Criticism and flattery are perfect opposites, yet they bring out different meanings of "susceptible to".

I am so confused.

Your original sentence strikes me as ambiguous. I think I would like to see the full context before I made a definitive judgement. Even so, my initial reaction was that the author meant that the performer was likely to be more influenced or affected by criticism after a poor verses after a good performance.

MeggPhaggSiouxSo "X is susceptible to Y" could mean either:
1. The probability of Y happening to X is now higher.
2. Y now has a greater affect on X.
Criticism and flattery are perfect opposites, yet they bring out different meanings of "susceptible to".

Yes, I think that it is all context + common sense.