Marvin Harrison Question?

I have marvin harrison but im not sure if i should start him this week because of his knee. I have roddy white on the bench who i like a lot but he plays at 1 and harrison is at 4 so i cant wait to see if marvin plays. On free agents i can pick up either andre davis or dj hackett who play when harrison does. So should i play white and bench marvin, or keep marvin in and if he doesnt play which of those WR should i pick up, hackett or davis?