This is a bit of a loaded topic. I consider it loaded, because the meaning of "Major League ready" has a definitive definition for each of you, and, for all practical purposes, your definition is no better or worse than any other. Nonetheless, Daniel Descalso has slumped, Addison Russell has baggage, and Nico Hoerner hasn't played a game since April. While Carlos Gonzalez has stretched the bench for the outfield, Garcia is the tempting option for the infield. Should Garcia get a call-=up before September?
I referenced different meanings of MLB-ready above. Your definition may be "will he produce at a (league average, perhaps) rate.” It could be an ability to deal with the increased (velocity/craftsmanship) of MLB pitchers. It may be a personal opposition to Russell or Descalso, or anything else. The reality is, it's tough to judge in advance, with any level of accuracy, how any player will perform on the big stage.
For instance, when a rash of injuries wiped out the entire opening day outfield from South Bend, the Midwest League team had one place to look. Extended spring training. When Brennen Davis was called up, I'd have been ecstatic with a .250 with a homer by the All-Star Break. As long as he wasn't treated like the Minnow in Gilligan's Island. Since the call-up, Davis has been the team's best player, whether hitter or pitcher. I'm a strong proponent of the 30/30/40 theory in baseball. Do all the homework, and a player will still surprise you 30 percent to the upside or 30 percent to the downside. If you start believing you can see the future accurately, you get 40 percent on the upside or downside, instead of accuracy.
Garcia is an infielder. Perhaps as he develops he can take to the outfield, but the Cubs have been trying to get him to be a scratch defender somewhere, first. At the Triple-A level, he's a beast. He hits for power, but not to the exclusion of doubles. He strikes out, as well, at a currently normal level. At Double-A, he was a bit of a novelty. Once the power was deemed real, he was promoted, to see if the bat was a Double-A thing. Alas, no. He's hit Pacific Coast League pitching with a similar vigor.
One day, watching him hit, I saw some Jeimer Candelario in his stance. He's a bit quicker and more versatile than Candelario. Similarly, Garcia could post late 2018 Candelario numbers, or early 2019 Candelario numbers, and I wouldn't be surprised with either. My expectations for Garcia are that I have no specific expectations. He'll likely do somewhat well with get-me-over meatball pitches, and struggle with pitches with extremely good location and late life. Like most hitters.
Garcia is a fun story. Signed by the Cubs as an older player, he has the ability to explode one of the more absurd biases in the faux-scouting game. Ageism. If Garcia can hit, what difference does it make how old he is? The “(insert player here) is facing a make-or-break season because he's "old" is claptrap. Garcia is 26, but if he were 28, but was suddenly mashing upper-level pro pitching, where's the harm in the signing, or putting him in the Tennessee/Iowa lineup?
Garcia's roster status in Chicago comes down to your conviction more than his skill. He's learning in Iowa. This is the best run of pitching he's faced, and the learning curve still likely is continuing. Your conviction is how much of a burden Descalso or Russell is on the roster. In your world, your conviction may be more important than Garcia's learning curve.
His defense has good days and those that are less good. When talking defense, I mentally flash back to the Mike Freeman days at shortstop. When the Cubs used Freeman there, he was considered on social media an unacceptable substitute. Perhaps that was correct, and perhaps not. However, when the only acceptable levels are "Russell"or "Baez", few will rate acceptably. Freeman is/was better defensively than Garcia.
My recommendation with a player, regardless the level, is to have a 10-by-10 chart in your brain. Take as complete and copious notes on a game as you can, and transfer those notes into one of the 100 spots. If you arrange it "diamond fashion," you can put the good games at the top, and the "three strikeout, two error" games at the bottom. By the time you have 100 boxes filled, you can assess the player.
I don't have the diamond filled on Garcia. I don't think his defense in MLB-caliber enough for a trip to the Cubs yet, but I've been wrong, plenty. To get his defense to MLB-speed, he has to play in MLB games. However, patience isn't a strong suit when games are being sacrificed. Relievers from Des Moines have taught me that. Last September, teams loaded their 40-man rosters, both with relievers and hitters. By October, the Cubs were about out of useful hitters. Garcia's bat is his carrying tool. It might, or might not, be a league-average option come September. He deserves a look, but then, so do many players at Iowa.
My hunch is Garcia is called up once the Iowa Cubs season concludes. How well his first cup of coffee goes is as likely a random chance, as anything. I doubt he's ready to do enough to earn regular playing time yet. As I doubted Davis would excel in South Bend. I tend to distrust people who are certain of future results they shouldn't have any special awareness of. My guess on Garcia's first 100 MLB at-bats? 0.2 to -0.2 in wins above, pick your preferred method of determination. If he hits a homer at the right moment, you'll dig him forever. However, if it's a hard-hit 4-6-3, you might not. Baseball is a great game, in part, because you really can't tell, much of the time. Garcia's glove gives me pause, but he's working on that, as well. If the brass buys him over someone specific on the 40 man roster, do the thing. I'd wait with Garcia (and Dakota Mekkes) until the last box is checked. If it comes to that, here's to me being wrong.