For the second time in less than a week, the San Francisco 49ers have lost a player to retirement and this one might be more surprising than the first one. 

Chris Borland, a 24-year-old linebacker who just finished his rookie season in San Francisco, surprisingly announced his retirement on Monday. Borland joins Patrick Willis, who announced his retirement on March 10. 

It's too early to say whether or not the NFL is in trouble, but the trend of early retirement is something that should definitely concern the league. Borland and Willis -- who's only 30 -- join Maurice Jones-Drew (29), Jake Locker (26) and Jason Worilds (27) as players who have called it quits since March 1. 

Without a doubt, the most surprising retirement of the five came from Borland, who only had one year under his belt and was poised to take over for Willis as the leader of the 49ers defense. 

Needless to say, Borland's retirement sent shockwaves through the NFL.

Here's how several current and former players reacted on Twitter. 

Former NFL receiver Donte Stallworth thinks Borland's retirement is going to start a trend of early retirements. 

Stallworth also said he's going to make sure the NFL doesn't go after Borland's signing bonus. 

Borland earned $1,037,436 during his one season in the NFL, a total that includes a $617,436 signing bonus he got as the 49ers third-round pick in 2014. 

As for other players, most were understanding of his decision, like Rams defensive lineman Chris Long, Giants offensive lineman Geoff Schwartz and Colts long snapper Matt Overton

There were also guys like Russell Wilson, who played with Borland in college and made sure to wish him the best. 

And then there was Broncos linebacker Brandon Marshall, who didn't make many new fans with the tweet below. 

Of course, just because Borland's retiring doesn't necessarily we'll see a trend start and that's mainly because there's always going to be players like Seahawks linebacker Bobby Wagner, who will play through anything. 

Finally, here's a good example of why Borland is retiring: Because he doesn't want to end up like 29-year-old former NFL tight end Tom Crabtree. 

49ers linebacker Chris Borland is retiring after just one season. (USATSI)
49ers linebacker Chris Borland is retiring after just one season. (USATSI)