Q: Should I resubmit to the same journal a manuscript they rejected but suggested for modification?
My paper was rejected before review. However, the reviewer asked me to modify it, mentioning that he would be interested in evaluating it after the modification. So, should I resubmit it to the same journal? Also, should I send the modified manuscript to his mail or submit as new?
I see your manuscript has received a desk rejection. However, even while rejecting it, the Associate Editor (AE) seems to see some potential in it.
If you wish to resubmit to the same journal, you will of course need to make the necessary modifications to it. Note that these changes are likely to be extensive, perhaps in terms of the experiments, methodology, or such. Therefore, you will need to spend a considerable amount of time on this. So, if you think this journal has rejected your manuscript for a variety of extraneous reasons (such as space constraints and volume of submissions) and that it can be accepted with some other journal in less time, you may consider submitting to another journal. However, note that with the other journal, you will need to go through the submission and decision process all over again. Now, this can happen with the present journal too, once you make the modifications and submit the manuscript again. So, you will need to decide based on these factors.
However, what I would suggest is that, if the AE’s suggestions seem valid, begin with making the modifications. Once the modified manuscript begins looking more robust, you could take a call on whether to resubmit to the present journal or try for another journal. Meanwhile, you could also identify other relevant journals and make a presubmission inquiry with them.
Finally, to answer your second question, the modified manuscript will be treated as a new submission. However, as needed, you may check the exact details of the submission process with the AE, either now (preferably) or at appropriate time later. All the best!