Looking at Burnley's fixtures in both the Championship and FA cup they will be playing away from home but their opponents are not that difficult... meaning a lowly team to say the least. Portsmouth is a team that is at the bottom of almost relegation so it is likely that Burnley can win.

While in the FA Cup Burnley will face Southampton in the Premier league this team is not so impressive losing more than winning so the points are very far with other teams.
But I see Southampton cannot be underestimated because if they compete with Burnley then this will be an equal match.