I've found a solution to morale in nearly every FM game I've played (12,14,17,18,20,21) where it might not work 100% but it has worked very well.
* Do not let your praise be over the top unless you won by a huge margin (more than 2 or 3 goals) and conceded none. Use the normal "Nice you did well" instead of delighted even if it was against your rivals.
* Always be harsh at half time even if you're leading 3-0 and ask the team to not get complacent. Reserve positive comments for the end of the match. Ignore anyone who gets frustrated because either you babysit one prima donna or you make sure the team wins.
* If it's away, be a little understanding and lower criticism especially if you lost with a goal difference of 1 or 2. At home, depending on how worse it was, you can be angry.