Witten suffered a "very serious" small laceration that caused bleeding but it is not ruptured, Cowboys coach Jason Garrett said Wednesday afternoon.
Garrett said it is premature to discuss Witten's availability for the Sept. 5 season opener against the New York Giants but confirmed the seven-time Pro Bowler will miss the remainder of the preseason.
Witten will be re-evaluated in a week to 10 days, and surgery remains a possibility if there is no improvement.
"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. You won't see him out there 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. You can imagine spleens are internal organs, and they're serious injuries involved with those things. Asking Jason Witten to be still and idle is a hard thing. That's the biggest challenge right now, but he has to do that. He has to let time heal it."
Witten briefly was on the practice field during Wednesday's walk-through but returned to the locker room before it ended. He is expected to stay with the team in California.
The Cowboys will travel by train Friday to San Diego for Saturday's preseason game and will practice against the Chargers for two days before returning home on Aug. 22.
"He's looked as good as I've seen him look since I've been around him," Garrett said. "I don't think 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 and be fresh and ready to go."
With 11 days off between the Cowboys' first two games, Garrett acknowledged the possibility that Witten could benefit from further rest but said it would not be a determining factor as to whether he would play against the Giants.
"Ultimately what factors into it more than anything else is his health," Garrett said. "We're not going to rush anybody back before they're ready to go and we'll just make those evaluations with him and anybody on our team prior to that."
Witten suffered the injury after taking a hit from Oakland Raiders linebacker Rolando McClain in the first quarter Monday, multiple sources said. Witten missed a few plays but returned and caught one more pass before the first-team offense called it a night.
Witten said he was OK after the game, but Cowboys owner and general manager Jerry Jones admitted some concern when he saw the two-time All-Pro leave the contest.
Losing Witten would be a major blow to an offense that relies on him heavily in the run and passing games. He has become the go-to guy for quarterback Tony Romo, catching 409 passes for 4,824 yards and 21 touchdowns since Romo took over as the starter in 2006.
"I know Jason is down because he hates missing anything football related, and that is part of his greatness," Romo said. "We're lucky to have a guy like him. He's an example for everybody.
"He'll be here with us fighting. When he gets back on the field, his ability we'll feel."
If Witten, who signed a six-year extension in September 2011, cannot play against the Giants, the Cowboys will look to John Phillips as the starter.
Phillips has missed the past week with a sprained ankle and did not play against the Raiders. He has 22 catches for 163 yards and a touchdown in 32 regular-season games. James Hanna, a sixth-round pick, caught two passes for 15 yards and Andrew Szczerba had one for 20 yards against Oakland.
Witten has missed one game in his career because of a fractured jaw suffered in his rookie season. He is 55 receptions away from passing Hall of Famer Michael Irvin as the franchise's all-time leading receiver. In nine seasons, he has 696 catches, third-most by a tight end in league history, for 7,909 yards and 41 touchdowns.
To help with the tight end numbers, the Cowboys will add Harry Flaherty to the roster and have waived/injured guard Bill Nagy.
Flaherty is the nephew of coach Jason Garrett and tight ends coach John Garrett. He worked out for the Cowboys last December. He went to camp last year with New Orleans and also can long snap.
Nagy, a seventh-round pick last year, suffered a high left ankle sprain in the first padded practice of training camp and is scheduled to visit a foot and ankle specialist in Charlotte, N.C., this week. If he clears waivers, he would revert to injured reserve. He started four games as a rookie before a fractured left ankle ended his season. The Cowboys had hoped he could compete for the starting center spot with Phil Costa.