I hate this tactic when valuable players hold out.
First, it means they are already contracted for that particular season so they are violating the terms of their present contract.
If a team does not extend a player's contract prior to its expiration, then that player can demonstrate his displeasure by signing with a different team as a veteran free agent.
Holding out in training camp is a clear threat, not only to the owner but to the players, coaches and fans.
He is basically saying:
"Despite being under a signed agreement for this season, you will pay me this much more money for future services. If you don't, then I will destroy your season."
“I think it’s important to let him know his teammates are thinking about him”
And yet, the real pressure is based on the results of any regular season game that Lamb may miss. Those fellow Cowboys receivers that have Lamb's "back" are the same ones that will try their utmost to prove that Lamb is not irreplaceable.
Let's not forget that football is a game of competition, not only team vs. team but also teammate vs. teammate in terms of playing time and the size of their slice of the cap pie.
In 1993, Emmitt's greatest contract leverage came from the performance of the Cowboys without him. They started off the season 0-2. Had they gone 2-0, it may have been an entirely different story.
Of course, these circumstances are somewhat different today. The 1993 season was before today's version of free agency with a salary cap. The contracts of each player today affect the contract negotiations of their teammates. This is how an advantage in scouting and drafting ability is neutralized in the name of parity.