There are really two big question marks for MIN'd LB corps, MLB and WLB.
SLB you can get away with trash in the T2D. I think MIN has above average trash for SLB.. servicable to the point that it won't be a weakness.
With the DTs that MIN has, probably one of the best pairs in the league, it's vital for a MLB to be a playmaker, because he'll be in position to do the most damage on the team. They have a big question mark there IMO.
In most variations of the Tampa-2, the MLB is the quarterback, but not necessarily the playmaker. Most variations scheme to spill plays to the weak side of the field, where the Mike and Will can both flow to the play. The Mike drops in coverage on passing downs, so the Will is more likely to be in position to make plays in the short zones.
And for WLB, you need all the speed you can get. I don't see Henderson beating out Greenway for this job when he struggled to win it before against a lower level of competition, and especially when MIN isn't going anywhere.
That was then, this is now. Insiders are suggesting that Henderson may finally be showing all of the skills that made him a highly regarded backer coming out of college. With Greenway having been more or less handpicked (Sims was probably their first choice) to play the Will, he'll be in there soon enough, but no team considers themselves out of contention in preseason and Greenway has to show he's not a liability in a complicated scheme before pushing what appears to be a solid vet option out of the way.
It's not like he's a QB and you might break his confidence or do damage to him by throwing him in the fire. I'm fairly confident Tomlin will not hesitate to start Greenway, and merely wants him to earn the job in front of his teammates so that he commands a higher position of respect, since he will be expected to lead this unit for the forseeable future.
I might also be pimping him because he's one of my starting LBs in my IDP league
Finally, full disclosure... 
Greenway and Hodge were a great combo, but I always wondered if one made the other better. I think Greenway will be fine, although not Pro Bowl level, but I'm really starting to lean more and more towards Hodge being the better of the two Iowa LBs.
