Talk about an unexpected gut punch. Organized team activities (OTAs) are supposed to be a time for rookies and fringe players to make a good impression. For veterans it’s a way to stay in good graces with the bosses and get a few workouts in. Season-ending injuries rarely occur in the Spring, but Michael Crabtree tore his Achilles — an injury that comes with an extraordinarily long recovery time.

It’s a tough blow for Crabtree, whose career was taking off and surely looked forward to a full season with a quarterback he actually liked working with for once. But life goes on for the 49ers, who can’t go into next season hoping Crabtree will come back in five months like Baltimore Ravens linebacker Terrell Suggs. Here’s what they’re looking at right now:

If the recovery time ends up being six months, that would put Crabtree back on the field in … hold on … late November. However, that’s on the short end of things, and Suggs’ case is an exception to the Achilles rule.

Michael Crabtree SF 49ers pregameIn March 2010, four orthopedic doctors at Duke University published a study that reviewed the careers of NFL players after suffering a ruptured Achilles tendon between 1997 and 2002. The doctors concluded that only about two-thirds of players returned to the field.

Of the 31 players who tore their Achilles tendons, 21 came back at an average of 11 months after the injury. Those players saw a decrease in the number of games they played and a dip in production in the three seasons after their return.

Khalid Shirzad, one of the specialists who contributed to the study, said the average number of games played by the 21 players slid from 11.7 per year before the injury to 6.2. Shirzad said the data collected also showed that the players’ “power ratings” — what the researchers defined as a statistical measure of performance, ranging from passing and rushing yards for an offensive player to tackles and interceptions for a defensive player — decreased nearly 50 percent.

The surgical landscape in 2013 is different than it was three years ago, and Crabtree *probably* had his surgery performed by an outstanding medical team at Stanford. But 2014 is a contract year for Crabtree, and it wouldn’t make a lot of sense to rush back if doing so might jeopardize his future earnings.

The idea of Crabtree making a triumphant return in December or the playoffs provides a way for 49ers fans to cope with today’s news. However, the 49ers have to not just get to the playoffs without Crabtree, but figure out a way to win playoff games without a receiver who caught 20 passes for 285 yards and three touchdowns in three postseason games last season.

We simply don’t know if Crabtree has enough time to come back and contribute anything this season. What we do know is this injury occurred early enough to give Jim Harbaugh and Greg Roman plenty of time to adjust their offense to compensate for the loss of their No. 1 receiver, as well as several months for Colin Kaepernick to find a new favorite target. The first step is to look at the depth chart.

1. Anquan Boldin

The rest: Mario Manningham, Kyle Williams, A.J. Jenkins, Quinton Patton, Ricardo Lockette, Chad Hall, Marlon Moore, Joe Hastings

Williams and Manningham are both returning from ACL tears. Jenkins didn’t catch a pass in his rookie season (but he gained four whole pounds this offseason, and if we assume it’s muscle I guess that’s something). Jim Harbaugh’s fun anecdote about Patton, along with the fourth-rounder’s production at La. Tech, have led to expectations for the rookie receiver that are probably unfair at this point. The other guys are all journeymen hoping for a chance.

Blech.

As Niners Nation points out, the list of available free agents is filled with egomaniacs and retreads (including Randy Moss, feel free to choose which category he goes in). The 49ers need to move the ball with the guys they already have under contract, which means they’re probably going to have to depend on skill position players who aren’t wide receivers more than most — if not all — NFL teams in 2013. With that in mind, here are four players the 49ers may depend on to fill the void left by Crabtree.

Vernon Davis: In the two years before Harbaugh and Roman took over the offense, Davis caught 134 passes for 1,879 yards and 20 touchdowns. Those numbers dropped to 108 receptions, 1,340 yards and 11 TD over the last two seasons. Davis has the highest cap number on the team this season at $8.7 million, and the 49ers will need him to earn most of that as a receiver instead of a blocker.

LaMichael James: He caught five passes for 40 yards over four regular season and three playoff games. The 49ers were probably planning on expanding his game and putting him in more spots on the field in his second season anyway, and now they have even more reason to get creative.

Vance McDonald: McDonald is smart enough to pick up the offense quickly. If he can put his knowledge to good use, run solid routes and get open, he’ll get targeted. A lot.

Garrett Celek: We could definitely see times when Davis is used as the primary deep threat, with Boldin, McDonald and Celek all working underneath at the same time.

The offense could look more Stanford-ish next season than at any other time since Harbaugh was hired, with 3-TE sets becoming commonplace and Roman counting on the best offensive line in the league to propel an offense that runs to set up the pass. Remember, even though the 49ers lost a star in Crabtree this week, they have another star in Frank Gore who refuses to let age and/or a high usage rate affect him.