Pre-Match Talks allow managers to further Motivate their Players ahead of the next big game, and they can have a great impact on the field of play. This is because of the brand new Dynamics feature included in Football Manager 2018, which will cause the players to have a more direct response from the actions of you, as the manager, and your style of coaching. Team Talks also help build the connection you will have will all of your players.
Unfortunately, however, if your manager doesn't have that great of a Motivating Attribute, the Pre-Match Talks with your team might not be all that effective. If this happens to be the case with where your manager is at, then it might be a wise idea to hand the duties over to your Assistant. If you decide to go this route, just make sure they have all the more Motivating Attribute apart of their coaching style. If they don't, they will end up being just as ineffective as you in regards to boosting the morale of you team heading into their next match.
But if you are dead set on getting your team motivated towards their upcoming match, then definitely consider giving your coach more of the Motivation Attribute when first creating them at the beginning of a new game. The higher value this stat has for your manager, the better these Pre-Match Talks will go. And the better you can convey your message to your team before heading out to the field of play, the better they will ultimately perform because of so.