You have to be cautious about making any judgments based on spring training, and it is a rule often cited among baseball professionals that you can't make any judgments about players based on spring training ...
I will also say that we with the Red Sox have made some terrible mistakes when we ignored this generalization. The worst was in Spring Training, 2014, when we brought in Grady Sizemore, who had once been a great player and who was only 31 years old, but had been out of baseball for a couple of years. Sizemore was incredible in spring training, absolutely incredible. He ran and moved fluidly, with great athleticism; he really was a beautiful athlete. Great stroke. In spring training every time his bat moved it was a line drive. We thought we had hit a home run; we thought we had picked up a great player for nothing.
Our plan had been to look at him in spriing training and then send him out to work himself back into top condition, but he was SO incredible in spring training--by far the best player on the team--that we wound up starting the season with him playing right field.
But once the bell rang he couldn't do anything. We said at first that it was cold weather, but the cold weather hung on and hung on. That decision blighted the season--not that we didn't make any OTHER mistakes that year; we certainly did, but that was a big one. We had won the World Series the year before, but Ellsbury left as a free agent; we had Jackie Bradley Jr. in center, and he was struggling as well, so we just weren't getting any runs out of our outfield. We wound up losing 91 games.
[James mentions Cesar Crespo needing to up his BB's, doing great in Florida, and then the moment they hit the Bright Lights he started fishing again worse than ever ... ]
But you have to be careful about generalizations, including this one. You have to try to see what is real, and what is an illusion, and it's hard work. Sometimes the steps forward that a player makes between seasons are larger than the differences between the player in a slump and the player on a hot streak. You don't make decisions based on spring training more than you have to, but if a player shows up stronger than he was, in great shape, enthusiastic about playing, smile on his face, sometimes you just have to honor that, regardless of what you believe.