Mr.Ashok Kumar felt much better yester, so he could / was able to play the match.

Please explain with rules of grammar which underlined verb is correct?

Mr.Ashok Kumar felt much better yesterday, so he was able to play the match.

Since the could and able to are used for the possibility for something to have happened in such sentences, hence it can be quite confusing. However in this particular sentence was able is more appropriate since it implies that because the subject felt better, he managed to play the match, the usage of could would imply that the subject got better in order to play the match. Hence was able is the correct form.