"When guys start making mental mistakes, doing things they wouldn't normally do, to me that's a little indication they're fatigued," Maddon said. "Or just look how many games they've played in a row sometimes."
Maddon cited B.J. Upton and Carl Crawford as examples of how he approaches resting players.
"You look at B.J., we're going to have to really keep an eye on that because you look at the end of last season, I thought his numbers came down offensively because he got tired," Maddon said. "Whether he wants to agree with that or not, I thought he played a lot of games last year. He was not used to it. So I think you need to keep an eye on him.
"Carl doesn't need a lot of time off, but when we double down like we did the other day [rested him Wednesday prior to an off-day on Thursday], it keeps him fresh. His legs are such a big part of his game. You want to keep him fresh."
Maddon also talked about Dioner Navarro, who has been a nice surprise offensively this season.
"Obviously we're playing him more and we're pushing him a little bit," Maddon said. "I had a little talk with him today about that specifically. 'I need to pick my days to give you off, but I need from you how you're feeling sometimes.'"
Following his diagnosis of mental mistakes serving as red flags, Maddon said he was concerned about Navarro's passed ball Sunday.
"But all it was just a mixup on the pitch," Maddon said. "He thought it was going to do one thing and it did another."
Maddon surmised the Rays just need to monitor the players every day.
"Because I do believe this, I believe you have to rest your guys during the season," Maddon said. "You have to. It's a long year and you want to play well in September. If you play well in September you'll do fine in October. You've got to keep them fresh for that good September. ... You just have to rest people at the right time and you've got to keep an eye on them. And it does matter at the end of the season."
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.