"So I went home for a couple weeks to let my body heal," said the 24-year-old first baseman. "The first week, I didn't do much of anything. The second week, I got back into the cage a little. I hoped it would be beneficial."
At first, it didn't seem to be. It was pretty much the opposite.
In his first Surprise Saguaros game on Oct. 9, he went hitless in four at-bats and struck out twice.
"That first day, it was like Little League out there. It was pretty ugly," he said. "I called back home, trying to figure out what was going on.
"The second day, it was a little different. I ran into some balls and began to recognize some pitches, like, 'Hey, I remember this.' It was coming back to me and I started to feel a little more comfortable."
The second day, the left-handed hitter went 4-for-5 and fell a triple short of the cycle. It propelled him to further heights during Week One, and he went on to become one of the AFL's co-Players of the Week, with first baseman Jonathan Singleton (Astros) of the Mesa Solar Sox.
The 6-foot-1, 210-pound McGuiness had a .389 batting average and an .833 slugging percentage for the week. In four games, he hit two home runs, two doubles and drove in six runs.
"I'm looking forward to seeing how the rest of [AFL season] goes, playing with and against some of the top prospects," McGuiness said. "Every day you face a top-of-the-line pitcher. Everybody seems like they throw 95."
It will be another grind, six games a week with Sundays off.
McGuiness is certain he can handle the situation. He learned discipline and physical and mental toughness at The Citadel (Charleston, S.C.) with a military-type atmosphere. He was a first-team Academic All-American in 2009 before becoming a 13th-round Draft pick by the Boston Red Sox later that year.
He had been with the Red Sox organization a little more than a year when he faced his first big test. They traded him to Texas on July 31, 2010, in a deal that sent catcher Jarrod Saltalamacchia to Boston.
"I was having a great time, making what I thought was pretty good progress, playing near home, when they told me, 'You've been traded,' " McGuiness said. "I thought they were joking. They said, 'No, seriously, pack your stuff up.' It was a big-time shock."
When the shock wore off, McGuiness approached the new challenge with optimism.
"Texas has one of the best farm systems around, and if you can do well there, you can do well anywhere," he said.
It wasn't, however, until he reached Frisco in 2012, that he began to put up numbers that drew attention -- .268, 23 homers, 77 RBIs, 122 hits and 65 runs scored in 123 games. But he is here now and that's all that matters.
"I wouldn't say I'm a power guy," he said. "It will go out if I hit it good, but I don't try to hit it out. If you keep worrying about that, it can mess your swing up. I just try to stick with what I know I can do, try to control the strike zone."
The Rangers do have a lot of depth, as McGuiness said. He doesn't know where it all will lead, but he hopes to do well enough in the AFL to perhaps earn a spot on the 40-man roster.
He knows that defense also is part of the equation.
"I've worked a lot on my defense this year," he said. "That first-step quickness, general agility, getting better reads, plays in the hole and picks in the dirt."
And if there appears to be a traffic jam at first base, he doesn't want that to be the end of his journey.
"I am willing to play corner outfield if that's what it takes," he said. "I just want to stay healthy out here, not necessarily look for any specific numbers. Just let the results come."
Don Ketchum is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.