It is true the police are obligated to turn over all evidence, whether it proves a person guilty or can be used for their defense. Nonetheless, the police couldn't testify at trial as to what the witnesses said. The witnesses would have to personally testify in the trial anyway for it to not be hearsay. The defendant's lawyer should conduct depositions of these witnesses to know what they would say. The fact that a police officer failed to include their statements in a report doesn't prevent the witnesses from testifying to it.
This is not to be considered legal advice nor does an attorney-client relationship exist.
If your lawyer found out about the interview after trial a motion for new trial based on newly discovered evidence can be filed. Speak with your trial attorney on this issue
I agree with what has been said so far but wish to clarify somewhat. The prosecutor is required to turn all statements which have been memorialized in some form to the defense, i.e., written, recorded, etc. The prosecutor also is required to turn over the name of any witnesses or anyone who has knowledge of the case. In general the prosecutor is charged with any knowledge the invistigator has whether the prosecutor is actually told of the statement or not. It would seem from the question that there may be a production violation either because their is a memorialized statement which was not produced or the name of some one with knowledge of the case was not produced to the defense. For a remedy to exist is such a situation though, there must be some actual harm to the defendant's case. While you seem to think there was, you need to consult an attorney experienced in the area to see if the statement would have actually made a difference in the outcome of the case, and if so what can be done to remudy the situation. Time may be of the essence since some issues are easier to deal with shortly after a trial but become more difficult or even precluded from remedy as time passes. Consult Counsel.