Rams acquire receiver Sammy Watkins in a trade for cornerback E.J. Gaines

On the eve of their preseason opener, the Rams and new coach Sean McVay decided to go deep.

The Rams traded for Buffalo Bills wide receiver Sammy Watkins on Friday, giving McVay and quarterback Jared Goff a desperately needed vertical threat.

The Rams sent cornerback E.J. Gaines and a potentially high-value second-round pick in next year’s draft to the Bills in exchange for Watkins and a sixth-round pick.

“Clearly you’re getting a special receiver,” McVay said after practice. “In his career, when he’s been available, he’s been outstanding in terms of being able to stretch the field vertically.

“When the ball’s in his hands, good things happen.”

Watkins might be on the sideline for — but will not participate in — Saturday night’s preseason game against the Dallas Cowboys at the Coliseum.

Regardless of when Watkins makes his Rams debut, McVay said the addition was helpful to the team, “and especially our quarterback.”

The 6-foot-1, 211-pound Watkins has been spectacular at times, but he also has been plagued by injuries since the Bills traded two first-round picks to move up and select him with the fourth pick in the 2014 draft.

Watkins, 24, has 153 career receptions for 2,459 yards and 17 touchdowns. He has averaged 16.1 yards per catch.

But he also has had hip surgery, two foot surgeries and other injuries. Last season, because of a foot injury, Watkins was limited to eight games. He caught 28 passes, two for touchdowns.

Uncertainty in the aftermath of Watkins’ injuries caused the Bills to pass on exercising their fifth-year option, which would have cost about $13.3 million. Watkins will earn $3.1 million this season in the final year of his rookie contract.

Rams general manager Les Snead said the Rams had been “flirting with Buffalo and Sammy” since spring. The talks intensified after the Bills declined the option on Watkins and the Rams declined to exercise their option on since-traded offensive tackle Greg Robinson, the No. 2 pick in 2014.

Watkins caught four passes — including three in the first three plays — on Thursday night in the Bills’ 17-10 exhibition loss to the Minnesota Vikings.

Snead said the Rams studied Watkins’ injury history.

“When you dove into it,” he said, “we’re comfortable taking the risk.”

If the Rams do not sign Watkins to a long-term contract, he will become a free agent at the end of the season.

Is this a long-range move for the Rams?

“He’s 24,” Snead said. “You don’t just do it for the now.”

Watkins joins a receiving corps that includes former Bills teammate Robert Woods, Tavon Austin, Pharoh Cooper, Mike Thomas and rookies Cooper Kupp and Josh Reynolds.

He provides the Rams with a proven deep threat, an element sorely lacking during training camp, and one that is important to McVay’s offense.

The speedy Austin and the rangy Reynolds have been sidelined because of hamstring injuries. Thomas is suspended for the first four games of the regular season because of a violation of the NFL’s performance-enhancing substance policy.

Snead said the Rams have been in the market for a deep threat since the offseason.

“That was on the agenda whether it was this spring, this summer, next free agency, next draft,” he said.

Last season with the Washington Redskins, McVay called plays for an offense that featured receivers DeSean Jackson and Pierre Garcon. Both eclipsed 1,000 yards receiving before signing as free agents with other teams.

“You look at some of the success Washington’s offense was able to have with some of the playmakers they have,” McVay said. “We’re trying to do something here.”

Woods played alongside Watkins for three seasons before signing a free-agent contract with the Rams that guaranteed $15 million. Woods said it was a “shock” when McVay informed the receivers of the trade Friday morning, but that he was excited to play again with Watkins.

“He’ll make it quite easier for us,” Woods said. “His vertical presence, and even just his threat and his name will allow defenses to play us different, because you always have to have someone over the top of him, which allows us to just have our one-on-one matchups.”

Gaines was starting his fourth season with the Rams and was on track to back up Kayvon Webster. He had a solid rookie year in 2014 but missed the 2015 season due to a foot injury. Last season, he played in 11 games but was slowed by several injuries.

McVay said that cornerback Nickell Robey-Coleman could move from the slot to outside if necessary and that Troy Hill also was capable.

Etc.

Linebackers Robert Quinn and Mark Barron, Thomas and possibly cornerback Kayvon Webster are among players who will not play Saturday as a precautionary measure, McVay said. . . . Star defensive lineman Aaron Donald remains a holdout. “Not any movement.” Snead said. “There is hope. There is hope that he’ll be a Ram a long time.”

gary.klein@latimes.com

Follow Gary Klein on Twitter @latimesklein

UPDATES:

9:45 p.m.: This article has been updated with additional details.

1:55 p.m.: This article was updated with additional Sean McVay and Les Snead quotes.

1:25 p.m.: Additional information added.

11:45 a.m.: This article updated with more information about Watkins.

This article was originally published at 10:15 a.m.