Recommendation letters from outside of your collaboration circle (your professor, your Ph.D. advisor, your manager, people you co-authored, etc.) are independent recommendation letters. Independent letters are deemed to be unbiased evaluation of your contributions and thus they could be of greater value in the eyes of immigration officers. For this reason, it is helpful to have them. However, there are cases where all recommendation letters are from the applicant’s collaboration circle. The bottom line is, getting them if you can. If you could not, it is also OK.