Keenan Allen: 'It won't happen again'

San Diego Chargers rookie wide receiver Keenan Allen apologized Monday for posting a photo of himself online wearing an Oakland Raiders cap, saying he didn't think it would cause the stir that it did.

"Obviously I had a Raiders hat on. I didn't think it would have the effect that it did have but obviously it did and I didn't mean for it to happen," Allen said sheepishly about getting caught wearing gear from the Chargers' most-hated rival.

"I wasn't thinking about it at the time. I wasn't actually thinking about football at all. I was just going to grab a bite to eat. I posted something on Vine.

"I'm sorry about it and it won't happen again." Allen said he bought the hat while in college at the University of California.

He posted the picture sometime after being drafted by the Chargers 76th overall (third round) in the 2013 draft.

He also said veteran safety Eric Weddle "came up to me today and gave me the rundown -- don't do it again."

Allen missed the start of the Chargers' organized team activities last week while he was taking final exams. San Diego is working the receiver into its program slowly and is expected to increase his level of activity on Tuesday.

Information from The Associated Press was used in this report.