In respect to Dependent Origination, the Pali suttas state:
Vedanā, saññā, cetanā, phasso, manasikāro - idaṃ vuccatāvuso nāmaṃ. Cattāri ca mahābhūtāni catunnañca mahābhūtānaṃ upādāya rūpaṃ. Idaṃ vuccatāvuso rūpaṃ. Iti idañca nāmaṃ idañca rūpaṃ - idaṃ vuccatāvuso nāmarūpaṃ.
Feeling, perception, volition, contact and attention — these are called mentality. The four great elements and the material form derived from the four great elements — these are called materiality.
Feeling, perception, intention, contact & attention: This is called name. The four great elements and the form dependent on the four great elements: This is called form. This name & this form are called name-&-form.
Why do we think 'attention' ('manasikara') is included within 'nama-rupa'?
Note: I personally do not have a definite answer to this question, apart from personal speculation therefore I have asked the question for people to think about & answer.