The Motown Sports Revival makes a strong case:

http://motownsportsrevival.blogspot.com/2009/09/walker-to-remember.html

Thankfully, Walker didn’t play his entire career with the Rockies. The means we can eliminate unsubstantiated assertions on whether Walker was solely a product of thin air and look for ourselves. Walker started off his career with the Montreal Expos in 1989. In just his third full season, he finished 5th in the NL MVP voting with a 142 OPS+. He was even better in his fifth season with Montreal. He hit .322 with a 151 OPS+. He also blasted 44 doubles which led the NL. The Expos played at Olympic Stadium which was in the middle of the pack in terms of Park Factor which essentially means that hitters and pitchers were on equal footing. Before Walker ever stepped foot in Colorado, he was a gold glove, all-star with two seasons of at least a 140 OPS+. He was an elite defensive outfielder and an above average base-stealer making him the quintessential five-tool player.

Walker played 9.5 seasons in Colorado where he was one of the premier players in MLB. In 2004, he was traded to the St. Louis Cardinals. The Cardinals played at Busch Stadium which was also in the middle of the pack in terms of Park Factor. In his first season with the Cards, Walker produced a 143 OPS+. In his second season with the Cards and last in the majors Walker contributed a 130 OPS+.

Walker played in three parks. Two of the three were hitter-neutral parks. The other was, of course, Coors Field. He played in a hitter-neutral park before Coors Field and was a budding superstar. He played in a hitter-neutral park after Coors Field and was well above the league average in production. Clearly, Walker was not a great player because of Coors Field. He was a great player who played at Coors Field.