AJ McCarron -- Pre-Spends 1st NFL Check to Put Katherine Webb on Lockdown

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Former Alabama QB AJ McCarron must be planning on making major bank in the NFL based on the diamond engagement ring he put on Katherine Webb today.

Katherine broke the news on Twitter -- showing off the ice she says AJ custom designed for her.

Brent Musburger's infamous crush didn't say anything about how AJ popped the question, but AJ's mom Dee Dee Bonner tells TMZ Sports both families are happy and excited for the couple ... and already in "wedding planning" mode.

No offense to Kat, but we're guessing AJ's main focus right now is planning for his NFL future. He's expected to go in the 2nd or 3rd round in the draft.

There goes his signing bonus.