Desmond doesn’t play well and the team does play well
This situation is perhaps the most perplexing as the team could go multiple ways in this situation alone. If Desmond doesn’t play well (like how he was the first two months of the season), the Rockies could just stick with them in their lineup, like Bud Black did, to the chagrin of most fans.
They could also decide to trade for a first baseman before the trade deadlines (either the non-waiver deadline on July 31 or the waiver trade deadline on August 31). Some names that have been mentioned as possibly being on the market are Jose Abreu, Logan Morrison, Lucas Duda, Matt Olson, Justin Smoak, Joey Gallo, Justin Bour, and Matt Carpenter (who is a third baseman but has played some first too).
There are also players like Adrian Gonzalez and Hanley Ramirez, who are first basemen that are currently free agents, who the Rockies could sign.
The Rockies could also turn to somebody currently in the organization, like Gerardo Parra, Ryan McMahon, or perhaps even Jordan Patterson.
Desmond doesn’t play well and the team doesn’t play well
This is obviously the worst case scenario. The team would most likely be (or at least should be) sellers. But they would definitely not be able to trade Desmond.
They would have to decide whether or not they would want to run him out there everyday or give way to younger players, like Ryan McMahon.