In a stunning opening game at the 2016 Rio Olympics, Croatia pulled off the Group B upset by taking down Spain 72-70 on Sunday.

We've gone into the past three Olympics (2008 in Beijing, 2012 in London, 2016 in Rio) thinking the same thing about men's basketball. USA Basketball is the class of the world on the hardwood and they should be a lock to win the gold medal, but keep an eye on the Spanish. Spain is the only other country deep in NBA talent. Their roster has seven current NBA players, three former NBA players, and a draft pick (Sergio Llull) who could have come over and had an NBA career if he had wanted to.

That depth of top talent didn't matter to Croatia, who kept clawing back and never backing down to Spain throughout the first game of group play. Behind 23 points from Brooklyn Nets guard Bojan Bogdanovic and a game-saving block by Philadelphia 76ers rookie Dario Saric on future Hall of Famer Pau Gasol, Croatia got a big victory that could help it outlast one of the four presumed powerhouses (Lithuania, Spain, Brazil, Argentina) and find itself in the bracket portion of competition.

So what does this stunning defeat mean for Spain? Here are three things to know:

Spain is flabbergasted by Croatia taking them down. USATSI

Spain didn't do so hot in the group play in 2012

Granted, this is an entirely different Olympics and a lot of these key Spanish players are past their prime, but Spain can take some solace in its mediocre 2012 group play in London and not worry too much about the first loss to Croatia. Back in 2012, Spain opened their group play with a solid victory against China and a nice win over a scrappy Australian team. That's when things started to get dicey for a Spanish team that was supposed to be a lock for the gold-medal showdown with Team USA.

In its third game, Spain beat Great Britain by one point. Great Britain was in the Olympic field only because it was the host country. The British had a truly bad team that was able to take down only an even worse China team. And yet, Spain didn't really have great focus in the game and allowed the hosts to be in it the entire time. The next game, Russia upset Spain 77-74, which meant the Spaniards were destined for a collision course with Team USA before the gold-medal game. That is ... until Spain dropped its fifth game rather intentionally.

Spain barely lost to Brazil in the fifth and final game of group play, which put it on the other side of the bracket from Team USA. The Spanish would go on to beat France and get their revenge against Russia to earn a return to the gold-medal game. They ended up losing 107-100 to USA. Maybe they'll end up having a similar route to the title game this year, but this is not the start they were hoping for.

It's safe to be concerned that Juan Carlos Navarro and Rudy Fernandez are no longer weapons

Juan Carlos Navarro used to be the most feared weapon for the Spanish team -- even more so than the Gasol brothers. His ability to be a constant 3-point threat from deep that allowed him to set up the rest of his game in the middle of the floor was truly special. Rudy Fernandez used to be the J.R. Smith of Spain. He had incredible talent, made incomprehensible gambles with his play at times, and could either shoot you out of a game or make you dominate with his spectacular presence on the court.

In the first game in Rio, neither player was a real factor. The 31-year-old Fernandez played 29 minutes and took only three shots in his time on the court. All three of his attempts were 3-pointers and all three of them missed. He was a scoring threat who didn't score a single point in nearly 75 percent of the game. Navarro is 36 years old and looked like he was a hologram out on the floor. The man they used to lovingly refer to as "La Bomba" played only 11 minutes and missed both of his 3-point shots.

Chicago Bulls forward Nikola Mirotic was the weapon complementing Pau Gasol -- not Navarro nor Fernandez. Maybe this was only one game, but their inability to hurt Croatia was certainly concerning.

Spain definitely misses Marc Gasol, which will likely keep it from challenging Team USA

Marc Gasol's absence this summer was something to worry about heading into Rio, and it was felt greatly against Croatia. Spain was outrebounded 41-30 and gave up 10 offensive rebounds. Dario Saric was all over the offensive boards, and Darko Planinic added three offensive rebounds. These possessions that gave Croatia extra life contributed to the upset victory and left Spain scratching their heads a little bit.

Croatia was a plus-14 with Planinic on the floor, and he really shouldn't be able to do much against Pau Gasol, Nikola Mirotic, and even Felipe Reyes and Willy Hernangomez. And yet, he was a big impact player. That doesn't happen if Marc Gasol is in Rio playing for Spain. Defensively, he was obviously missed and he's not a big time rebounder. But he's a giant body on the floor and one who takes up space and can be effective on the glass against the opposition -- especially against a frontcourt like Croatia's, which shouldn't be able to outperform Spain on the boards.

His absence probably keeps any potential gold-medal game against the U.S. from being as close as we saw in 2012. Remember that USA Basketball is missing LeBron James, Stephen Curry, Russell Westbrook, Anthony Davis, James Harden, Blake Griffin, Chris Paul and others. This isn't a B-Team by any means, but there's enough of a dip in talent for Team USA that a full-strength Spain team could've shocked the world and accomplished what Argentina did in 2004.

Instead, Marc Gasol isn't there and Team USA will be playing its best when it gets to the gold medal game. Will Spain be playing its best? Will Spain even be playing in that game?