...Michael is the Big Don and Frankie is the little Don, Frankie realized that and, again, he was too much of an old soldier not to follow orders until he thought that Michael tried to kill him...
True...and if that's all there were to it then there would've been no need for Michael to pay this surprise visit to Frankie in the first place.
Frankie had already conceded that despite his need to vent his disgust and distrust of Roth, that there would be no trouble from him. That meant he would do what he was told...but wouldn't like it. Michael knew that was true when he allowed Frankie to go home ("The old man had too much wine...")
He DID need to manipulate Frankie and that's exactly what he did do in this scene.
And by the way, the manipulation begins right here (if not prior to Frankie even entering his own house):
FRANKIE: Don Corleone, I wish you had let me know you were coming; I could've prepared something for you.
MICHAEL: I didn't want you to know I was coming. 
