The middle linebacker almost invariably has the microphone implanted surgically in his head (citation needed) that enables him to hear the coach pre-snap and adjust the coverage. Mason Foster is going to have an inadequate amount of preparation to be relied upon with such an important task, but Raheem Morris is likely to throw it at him anyway because he'll have to get used to it at some point. The main concern is that, as MLB, the onus is on Foster to read the offense and react accordingly. He's needed to direct the three levels of coverage on the basis of what he hears from the coaches and what he gleans from the oppositions setup. If Foster proves disastrously unable, the helmet decor may lie elsewhere- for at least this season.
Geno Hayes and Quincy Black would be leading candidates for a variety of reasons. They've got a few years under their belt and play at a part of the field with a good enough view of the offense. Unfortunately as a Will/Sam they've never had that responsibility, and the learning curve may be a bit too steep for a younger player in such a short time. Cody Grimm lacks the familiarity to step up to the plate in this instance as well. Ronde Barber is by far the most senior player with an absolute understanding of the team, however as CB he's working with insurmountable obstacles. Barber will often play close to the sideline, and that will leave him out of position to optimally see the offense while being out of earshot for other defenders to make his adjustments.
Ultimately, if the MLB can't handle pre-snap reads, the best bet may be Sean Jones. Jones has had a full camp and season with the team. He's going into his eighth season (the amount of experience combined of the starting LB's and other S) and has enough familiarity to read the offense better than the four contenders who play in front of him. Jones sits deep in the defensive backfield allowing him to see everything on both his side of the ball and the offense's. He can monitor those who he reassigns without having to turn his back to the play, and isn't too far for the whole field to hear him after stepping in. Furthermore, Jones will always be on the field. If Morris takes Foster off the field on passing downs as he's planning on for the preseason, it would be highly impractical for him to have the mic.
Foster isn't going to lose his starting job outside of injury unless he redefines 'awful'. Morris is probably going to stick with him using the microphone while Barber, Jones, and the other LB's lending a helping hand on the field. Should Foster frustrate Morris too much, look for Jones to get a lot more responsibility.