Based on Shoe's answer (the most upvoted answer) lesser can only be used attributively. So you can't say:

The pain is lesser. You can only say a lesser pain.

Consequently the correct use would be:

The pain is less.

Based on Shoe's answer (the most upvoted answer) lesser can only be used attributively. So you can't say:

The pain is lesser. You can only say a lesser pain.

Consequently the correct use would be:

The pain is less.