The return of the NFL’s reigning Most Valuable Player coincided with a drop in performance among Kansas City Chiefs defenders.In the two-plus games quarterback Patrick Mahomes missed with a dislocated kneecap, the Chiefs stopped the run effectively and did not give up a 100-yard performance. Their defense held its own.When Mahomes returned last week, the defense contributed to a total team collapse. The unexpected letdown tightened the AFC West chase as Kansas City (6-4) meets a division rival, the Los Angeles Chargers (4-6), on Monday night at Mexico City.”You can’t change what happened,” Chiefs safety Daniel Sorensen said. “What you can do is focus on fixing and improving as we go down this last stretch.”The closing run is tighter than most predicted. The Chiefs are tied in the loss column with the Oakland Raiders (5-4) atop the AFC West, and the Chargers can tighten things even more by beating Kansas City on a neutral field in prime time.”We’re in a battle now,” Mahomes said.The matchup is a second crack for Kansas City to play in Mexico City after field conditions last year at Estadio Azteca prompted a scheduled appearance against the Rams to be moved to Los Angeles.”I was hopeful that we would have a chance to go to Mexico City,” said Chiefs chairman Clark Hunt, noting that the league assured his staff that the turf is much improved. “All of the international games are very important for the league, and they are also important for the Chiefs from a branding standpoint. I frankly did not expect that we would be (scheduled again for) Mexico City so fast.”Kansas City is designated as the visitor, leaving the Chiefs with three home games, one each against division rivals, and road tests at New England and Chicago. The Chiefs’ strength of schedule currently ranks fifth.Expect the Chargers to try to establish a balanced attack. Melvin Gordon, coming off a 108-yard performance in a loss to the Raiders, will try to exploit a Chiefs run defense that ranks 31st.Kansas City allowed 188 yards last week to Derrick Henry, who led a Tennessee rally that capitalized on two botched field goals.