It's doubtful that at least one, possibly all of those players, will be starting when the season rolls around. I'd guess that at least a midtier lineman will be acquired in FA if neither Melton or Allen sign, then Dallas is going to draft a DL in the first round without a doubt and possibly the second round, too, possibly moving up for one if necessary.
Trouble is, we need more than just one bona fide pass rusher in this defense. If the first-round pick is the only one who can get consistent pressure we'll be in trouble. (Not counting on a second-rounder being bona fide.)
If we don't get Melton or Allen, then we'll have to hope that Marinelli can turn the midtier FA and Bass or Crawford into consistent threats. (I don't think he can make Selvie any better than he was last year ... which was very good for a rotation guy, but not consistent enough with pressure for a starter.)