It's a reasonable question, and no clear answer. Comments mentioning seasoning make a bit of sense given Tyler Collins had a college career prior to turning pro, but Daniel Fields is the same age (23) as Collins, and was obviously considered important to the organization given his addition onto the 40-man roster last off-season (which the Tigers didn't feel necessary to do with Collins).
Nevertheless, Collins hit his third home run in big league camp on Tuesday, tied for the team lead, while Fields was optioned to Triple-A Toledo nearly a week ago, even before prospect Steven Moya was, even though manager Brad Ausmus acknowledged had no real shot of making the team.
So, what gives?
In watching Fields, one thing seemed clear, and that's that his swing appears to have gotten longer. Could just be a small sample of at-bats with Fields working on something in particular, but a long swing resulted in a swing and miss on a hittable fastball, and ultimately resulted in a flyout in Monday's intradsquad game.
Those struggles continued on Tuesday – Fields struck out twice, with a number of foul tips, in additional to quite a few swing and misses. In his final at-bat, where he worked a walk, he still swung and missed at two pitches, both outside the strike zone. It was no doubt a discouraging sign, and he showed similar in big league camp, it could be a reason why Collins quickly passed him in the outfield competition pecking order.
There are other explanations as well of course beyond just a handful of at-bats and number of years playing the game. Collins already has extensive experience in left field, where Andy Dirks would have played. It could also be a matter of Collins being closer to his ceiling, and the Tigers being higher on Fields' long-term potential. Fields was given the edge in TigsTown's 2014 Top 50, coming in number ten, while Collins ranked 14.
Basically, the real answer will probably never be disclosed, but any number of factors could have contributed. No matter the reason though, Fields struggling at the plate has been evident to observers, and it's worth pointing that out, especially after what amounted to a re-breakout season in 2013.