The last-place Colorado Rockies are losers of five straight and could return home with a depleted lineup.Zimmerman looks to continue his success against the Rockies on Monday night in the opener of a three-game set in Denver.The Nationals third baseman went 6 for 12 with a homer and five RBIs over the weekend as his club won two of three at home over Milwaukee. He's 11 for 26 with 10 RBIs during a seven-game hitting streak.Zimmerman is a career .348 hitter against Colorado (40-58) for his highest mark against any team. His .368 average in 29 games at Coors Field is his best in any ballpark.Rockies manager Walt Weiss, meanwhile, can't be sure who he will pencil into Monday's lineup.Troy Tulowitzki, who leads the majors with a .340 average, sat out Sunday's 5-3 defeat at Pittsburgh. He suffered a cramp in the tensor fascia latae of his left thigh while running out a grounder Saturday."He's sore," Weiss told MLB's official website. "I still consider him day to day, but he's sore. It could be a couple or three days. We'll monitor it and just check in with him hourly, but I'm expecting it to be another day or two."Colorado has other injury concerns as well.Justin Morneau missed a third straight game with a stiff neck while All-Star outfielder Charlie Blackmon sat out Sunday's contest with an ankle injury. Weiss indicated that Blackmon is more likely to return Monday.Those three players were all available June 30-July 2, but it didn't matter as Washington (53-43) swept a three-game set in which Zimmerman went 5 for 9.Nationals starter Doug Fister (8-2, 2.90 ERA) will pitch at Coors Field for the first time after going 2-0 with a 2.08 ERA in two previous starts against the Rockies. He probably doesn't want to see Morneau, who is 9 for 28 with a homer against him.The right-hander improved to 3-0 with a 2.57 ERA in his last four outings by limiting Baltimore to two runs over seven innings July 9 in a 6-2 road win.Colorado will counter with Franklin Morales (5-4, 5.26), who is 2-0 with a 3.45 ERA in his last five home starts.The left-hander won for the first time in eight starts July 8 by allowing one run over 5 2/3 innings in a 2-1 victory over San Diego.Morales has never started against Washington, throwing 6 2/3 scoreless innings in five relief appearances.The Rockies can't be confident even if Morales exits with a lead since their bullpen coughed up leads in all three weekend losses."You've got to be able to put games away, and it's frustrating," Weiss said.Washington has won 12 of 17. Jayson Werth's RBI double in the bottom of the ninth Sunday gave the Nationals a 5-4 victory."It's why we do this," Werth said. "If you find yourself in that situation and you don't want to be there, I think you're in the wrong line of work."