Weeks responded by knocking the first pitch from Colorado starter Christian Friedrich over the left-field wall. Weeks also singled in the third and laced a leadoff double in the fifth as the Brewers defeated the Rockies 7-4 Thursday night.
''You've got to do what you've got to do. I'm a professional, so you've got to do your job and be ready and go out there and perform,'' Weeks said.
His performance impressed Roenicke.
''The first pitch he sees, he just turns on it. A great swing. He had a good night swinging the bat,'' Roenicke said.
Gennett leads National League second basemen in batting average, slugging percentage and OPS, but found himself on the bench as the Brewers faced a left-hander in Friedrich.
Not to be outdone, Gennett smacked a two-run, pinch-hit homer in the eighth.
Wily Peralta (9-5) won his fifth consecutive start for Milwaukee, but wasn't sharp at times. He allowed three straight singles in the fourth before Ryan Wheeler hit a grand slam that gave the Rockies a 4-3 lead.
Peralta gave up four runs and eight hits in six innings. He struck out seven and walked two in his second win over the Rockies in less than a week.
''He can pitch better,'' Roenicke said. ''I thought the last three outings that he's pitched well, but he's not locked in. He's still making good pitches, but he's struggling to get through the lineups.''
Aramis Ramirez added a two-run homer in the first to stake the Brewers to a 3-0 lead.
In the fifth, Weeks led off with a double and moved to third on Ryan Braun's single. Weeks scored on Lucroy's single off the top of the wall in straightaway center. Drew Stubbs nearly caught the drive, keeping Braun from advancing beyond second base.
A sacrifice fly by Khris Davis put the Brewers ahead 5-4.
Francisco Rodriguez earned his major league-leading 26th save.
Friedrich (0-2) yielded five runs and nine hits over four innings. He struck out eight and walked one in his second start of the season - both against the Brewers.
''I was getting behind batters and not really executing down in the zone,'' Friedrich said. ''You just have to execute, especially when you're ahead.''
Friedrich temporarily settled down after the rough first inning, striking out four consecutive batters during one stretch - including all three in the second. But he ran into trouble again in the fifth, when the Brewers regained the lead.
''It's a tough recipe when your starter is out of the game before the sixth,'' Colorado manager Walt Weiss said.
The Rockies threatened in the seventh, loading the bases with two outs. Will Smith struck out Corey Dickerson to preserve Milwaukee's lead.
In the Colorado eighth, Michael McKenry led off with a double against Brandon Kintzler. After consecutive groundouts, pinch-hitter Brandon Barnes struck out looking, leaving McKenry stranded at third. Kintzler pumped his arms and yelled as he left the mound.
''It was just a matter of the frustration of the leadoff double,'' Kintzler said. ''Obviously, my whole season has been frustrating, so to walk into the dugout on a positive note, I let my emotions out.''
NOTES: Weeks hit his 26th career leadoff homer. ... Wheeler's grand slam was the first of his career and just his third home run in three seasons of limited action with the Rockies. ... Colorado catcher Wilin Rosario left with lower back tightness. ... Rockies reliever Boone Logan (left elbow inflammation) made a rehab appearance for Triple-A Colorado Springs. ... Milwaukee's Kyle Lohse (9-2) will face Tyler Matzek (1-2) on Friday in the second of a four-game series.