OXNARD, Calif. — Jason Witten is going to have to get used to not doing much of anything if he wants to play in the season opener next month.

Dallas Cowboys coach Jason Garrett said Wednesday that his Pro Bowl tight end sustained a "slightly lacerated" spleen in the preseason opener Monday night against Oakland. The best way for Witten to heal is to remain as inactive as possible.

"It's a very serious injury and we're really taking this thing day by day," Garrett said. "He has to be very still and idle for the next week to 10 days, so you won't see him out here on the practice field. He won't play in the game this week or next week. We've just got to make sure he heals as well as possible, as quickly as possible."

Witten is expected to miss the rest of the preseason, starting with Saturday night's game at San Diego. The Cowboys (No. 15 in the AP Pro32) open the season in three weeks, Sept. 5, at the defending Super Bowl champion New York Giants.

The coach said doctors will reassess Witten next week "after we've had a few days of him not doing anything." Surgery would be an option if the spleen isn't healing properly by then.

Cowboys owner Jerry Jones said after practice Wednesday that he hasn't even looked at whether Witten would be ready for the opener against the Giants.

"We basically were more interested in (the) rest of this week and getting that healed back up," Jones said. "It's diagnosed real good, with the cut on his spleen. He can't do anything or move around too much. And then it heals up like any cut, hopefully he will be back."

Witten apparently got hurt after he caught a pass from a scrambling Tony Romo and was immediately hit from the side by Oakland linebacker Rolando McClain. The play resulted in a 2-yard loss.

Witten has missed only one game in his nine NFL seasons. That was as a rookie in 2003 when he had a broken jaw.

"Asking Wit to be still and idle is a hard thing, that's the biggest challenge right now. But he has to do that, he just has to let time heal it," Garrett said. "Jason has gotten off to a tremendous, tremendous start to training camp and laid a great foundation for himself. ... He's looked as good as I've seen him look since I've been around him."

Witten is a seven-time Pro Bowl player who is one of Romo's best friends and one of the quarterback's favorite targets. The tight end has 696 career receptions, second in team history behind only Hall of Fame receiver Michael Irvin at 750.

Because Witten has started every game the past five seasons, and all 77 games Romo has started since becoming their No. 1 quarterback in 2006, the Cowboys aren't concerned about him missing some time in training camp.

"I don't think this, at the end of the day, him being away from our football team and off the practice field, is going to be that detrimental to him. In fact, I think it's going to allow him to get his legs back a little bit, and be fresh and ready to go," Garrett said. "We're going to spin this thing positively and say hey this rest will help him."

The Cowboys signed free-agent tight end Harry Flaherty, the nephew of the head coach and tight ends coach John Garrett. He signed with New Orleans as an undrafted rookie out of Princeton last season but was cut early in camp. To make room on the roster for Flaherty, the Cowboys waived injured guard Bill Nagy.

John Phillips, a fourth-year player, will likely fill in as a starter for Witten. Rookie tight end James Hanna will also get more work.

Phillips missed the game against the Raiders because of a sprained ankle but was back at practice Wednesday. He said the swelling was gone and that his ankle was good, plus talked about the difficulty of being without Witten.

"Can't replace a guy like that. I am sure he's going to do the things needed to get back. I am sure he will be ready to go when the season rolls around," Phillips said. "He's a leader of this team, a leader of this offense. Not trying to replace him. He will be able to lead from the sidelines. Help us out in film room."

Notes: Garrett said starting center Phil Costa, who missed the Oakland game with lower back stiffness, likely would not practice Wednesday or Thursday. Because Friday is only a walk-through, Costa's availability for Saturday's game at San Diego is questionable. ... The Cowboys are winding down their camp in Oxnard. After a walkthrough Friday morning, they break camp and head to San Diego. ... Rookie WR Danny Coale missed practice again Wednesday with a sore left quadriceps.