Your comment is again lockstepped into the mentality that another pitcher will be #5. Why? I'd rather have 5 #2's than a 1,2,3,4,5. Pitchers get hurt. When the season starts, your #5 may go against the other team's #1 many times because of a rainout or a schedule fluke. Gotta get rid of that thinking. It only works on paper; not in real play.
You are missing the point.
They now have four decent starters - Hendricks, Lester, Quintana and Chatwood. If they sign another starter, SOMEONE will become largely superfluous in the playoffs. Whether that is the new signee or Chatwood or Hendricks, or lester, or Quintana, it will add little value in the playoffs, And, in my opinion, they are likely to get to the playoffs even if they do not sign the new pitcher. And I doubt that they will sign a new pitcher who will so much better than Chatwood in the playoffs that it will justify the price.
What number you assign to the new pitcher is meaningless. The point is, he is probably not needed. It is true that depth is helpful. Montgomery provides that depth.