Thinking about who is in the minors to pitch...Francis Martes will be coming back from TJS and he pitched decent in 2017 during some spot starts, so he has potential. Cionel Perez is a AAA starter, so maybe they'll play with stretching him out in spring training. Don't forget about Armenteros, who pitched pretty well on occasion this year. Framber can't be trusted to throw strikes, but if you could fix that somehow, then he may be an option.
I wouldn't be surprised to see them use Peacock as a starter again. Before he had his nerve issues he was pretty solid.
Whitley has maturity issues that were well documented and he hasn't been able to deliver at a AAA level yet, so I can't see the astros using him before midseason. They tend to wait until the super two date passes anyway to give guys their first call up (see Alvarez and Tucker). At best he may be a long relief addition at the end if the year or, if he still isn't producing come July, they might cut the cord and trade him for whatever the team needs.
Keep in mind last year that there were projections for big deals for free agents about this time, and lots of guys took lower valued contracts with fewer years. Lots of the free agent arms will likely get tagged with qualifying offers, so we will see the same hesitancy to sign guys because it means giving up draft picks that are more highly valued than ever.
It will be interesting to see what the astros do given the financial limits that they have. Watch the rule five draft pickups closely and the waiver wires. We haven't heard of urquidy at all this time last year, and he came through big and is someone they're counting on to be a starter this year. I would bet they go after a low cost LH starter and a long reliever as insurance. Given the age of Grienke and Verlander, you can't count on them being injury free all year. Trade Reddick, Devo, Biagini, and a prospect for some pitching and clear some salary in the process to be able to sign some relievers.