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LANDOVER, Md. -- Given a chance to make a closing argument in his case to be the Washington Redskins starting quarterback, John Beck left the competition as muddled as ever.

Beck turned in a spotty performance Thursday night, completing 10 of 21 passes for 108 yards with an interception in a 29-24 win over the Tampa Bay Buccaneers in the teams' final preseason game.

"I obviously would have liked for it to go better than it did," Beck said. "I missed some throws out there that I usually don't miss on."

Coach Mike Shanahan said he will look at the tape and decide over the next few days whether Beck or Rex Grossman starts the season opener against the New York Giants on Sept. 11.

Right now, it's anybody's guess.

"What's going to happen?" Beck said. "I don't know."

Beck played the entire first half with the starting offensive line and backup running backs and receivers, while the Buccaneers gave all of their first-stringers the night off. The Redskins' offense produced 10 points in Beck's eight series, but the touchdown and the field goal both came on short fields after the defense forced a turnover. Grossman did not play.

Shanahan said the decision to mix starters with backups contributed to Beck's uneven performance.

"It's a little tough to go out there today, with a lot of different people playing a lot of different positions," Shanahan said. "Any time you don't go with your first units, and you try to separate people, wide receivers, running backs, offensive linemen -- sometimes it gets a little ugly."

The Redskins pulled out the win on Kellen Clemens' 41-yard touchdown pass to Donte' Stallworth with 1:07 remaining. The score came exactly one minute after the Buccaneers had taken the lead on Jonathan Crompton's 20-yard pass to rookie free agent Armando Allen Jr., followed by Crompton's completion to Ed Gant for the 2-point conversion.

Gant, a bottom-of-the-depth-chart first-year player, also had a 96-yard catch-and-run touchdown in the third quarter in his bid to be included in the 53-man regular season roster when the final cuts are made over the next couple of days.

"I leave it up for grabs," Gant said. "I just come to work, man. I think I did OK. My opinion is I did OK."

Beck and Grossman have been battling since the opening of training camp. Both played well through the first three preseason games, prompting Shanahan to call the competition "very close" earlier this week.

If that's the case, Grossman might have won just by standing on the sideline.

"There was a few throws there that were kind of like a short putt," Beck said. "When you're out golfing, you're feeling good, you're having a good day and you go to putt it and it doesn't go in, you're like 'Wait a second. I'm feeling too good. I'm feeling zoned in. Those things usually don't happen."

Shanahan said he sat Grossman because Beck had played less during the preseason. The coach rested most of his first-teamers on both sides of the ball, a mild surprise after hinting earlier in the week that many of his starters would play.

Beck made a couple of nice throws under pressure, but he also had two passes nearly intercepted in the first quarter. The touchdown drive he led was all of 2 yards -- and it took three plays because he fumbled the snap on first down. It didn't help that he didn't have Santana Moss or Jabar Gaffney as targets -- some of the incompletions appeared to result from miscommunication with the receivers.

Beck's most dubious lowlight came when he unwisely heaved a pass up for grabs while under pressure from a blitz on a third-and-6 from Tampa Bay's 15-yard line. D.J. Johnson made the interception in front of rookie receiver Niles Paul in the end zone.

Beck finished the preseason 30 for 48 for 356 yards with one touchdown, two interceptions and a 74.7 rating. Grossman was 34 for 53 for 407 yards with two touchdowns, one interception and a 92.3 rating.

It might not matter whether Beck or Grossman comes out on top unless the offensive line plays better. Perhaps the most worrisome development for the Redskins was the play of the front five, who frequently gave ground against Tampa Bay's backups and allowed Beck to get hit several times. Beck's nimbleness helped him avoid the pressure when he could, showing a mobility that could give him the nod ahead of Grossman.

"We were up and down," right guard Chris Chester said. "And there's definitely going to be room for improvement."

The Redskins' highlight came on a 95-yard punt return by Brandon Banks, whose roster chances have slipped this preseason because of a sore knee. The bad news for Banks was that he celebrated too early and let go of the ball at the goal line. The touchdown was confirmed only after a replay review, and Banks subsequently got an earful from Shanahan and special teams coach Danny Smith.

"I can't do that no more. I didn't realize I did that," Banks said. "It was a close call, and I'm glad it went my way."

Game notes

Washington RB Evan Royster left the game with a mild concussion, and CB Phillip Buchanon injured his right shoulder. ... Tampa Bay CB Myron Lewis injured an ankle. .... The Redskins started two first-teamers on defense -- CB Josh Wilson and DE Adam Carriker. Both have been limited this preseason because of injuries. ... Bucs coach Raheem Morris joked about the horrific traffic en route to the stadium. Police escorts for sports teams visiting the Washington area have been frowned upon since actor Charlie Sheen tweeted about getting one in April. "I guess Charlie Sheen messed it up," Morris said, "because we couldn't fly through the city like we used to do,"