Upon deciding to drop the case the CPS explained there was “no longer a realistic prospect of conviction” after key witnesses withdrew their cooperation from the investigation.
"Key witnesses" can only mean the victim. It seems to me that we have cause and effect the wrong way round here. Mason Greenwood is able to return to Manchester United because the victim (I'm not saying "alleged" because apparently there is a recording of the assault) has been silenced.
If I were in charge at Man U, and assuming the evidence is as strong as described in the article, I would be terminating Greenwood's contract, or at least not renewing it id that's legally problematic.