for what it's worth, i don't think morale makes any difference for single player. so if you have 10 players with perfect morale and 1 with very poor, the 1 is as likely to play well as the others. i think it works the same way quality works (i guess that's as much up for debate as morale) so the engine just adds up the numbers for your starting 11 and gives whatever result with your 3 star player scoring better than 7 star striker.
edit. actually in your case 1 star player vs 9 star player. by your example you could also argue that quality is irrelevant.