To win Premier League, Manchester United will have to beat both Manchester City and Arsenal. Honestly it is an impossible task for them but they can win against Newcastle United to get a 3rd position at the end. That position is a best one Manchester United can get in this season. It can be a foundation for them to aim at a better achievement in next season. A third position if they can get, will give them more confidence that they are competitive enough in Premier League. It can help them to buy some star players in the summer which are meaningful for their building process.
Theoretically, even Newcastle can win the Premier League - as we can see, they managed not to lose to Arsenal and City and, along with Arsenal, have only one defeat in the whole season. But the likelihood of Newcastle or United winning the championship is, of course, very small.
It seems to me that there is not much difference between third and fourth place - the most important thing is that a ticket to the Champions League allows you to hire ambitious players. So if Newcastle and United stay in the top four we will see their activity continue in the transfer market (especially I expect surprises from Newcastle).