The Patriots have a pretty important decision to make that’s about Tom Brady, but not directly. It’s about the backup plan for the Pro Bowl quarterback, which ultimately leads to his successor plan. Currently, that backup plan involves Ryan Mallett. The question is whether the Pats should continue to hold onto the fourth-year pro, who has just one career pass completion and barely any more snaps in his first three seasons, or dip into the draft this year and see if there’s another potential replacement available. Mallett is in the final year of his rookie deal. The Pats could re-up him, extend his period as Brady’s understudy, or perhaps trade him. Already, there’s been some speculation about him going to the Texans and rejoining Bill O’Brien given Mallett knows the offense the former offensive Patriots coordinator plans to install in Houston. He’s certainly a tradeable commodity. While he has yet to start a regular-season game and has seen only nominal mop-up duty in four games, the strong-armed quarterback has been mentored by Brady and Bill Belichick. That elevates his stock. Should the Pats simply let Mallett play out the contract, there’s the potential of losing him in free agency next year. The bottom line is the Pats need to stay on top of the curve when it comes to Brady’s backup and ultimate successor. You never know what’s going to happen. Just ask former Colts general manager Bill Polian. When we discussed the issue with him last week at the NFL scouting combine in Indianapolis, Polian said he got caught without a suitable backup when Peyton Manning was lost for the year and paid the ultimate price — his job. Noting that he failed to follow the example Ron Wolf set as the GM in Green Bay with Aaron Rodgers behind Brett Favre, he didn’t have a player available to reliably fill in for Manning when needed. Said Polian: “But I believe even if you’ve got that guy, you ought to draft one every two or three years, if you feel like there’s a guy out there that can play. Draft one every two or three years; get him ready. If you’re going to carry an older guy as a backup, fine, (the prospect) has got even more time to develop.” Without Manning, and without a legitimate backup, that ultimately led to an NFL-worst record in 2011 and the No. 1 pick in the following spring’s draft, which proved to be Andrew Luck. But that wasn’t exactly the plan. “That was just luck,” Polian said, pun intended. The Pats, of course, lost Brady for the year in 2008 when he blew out his knee in the opener, but they didn’t have such a miserable season. They didn’t make the playoffs, but 11-5 wasn’t too bad with backup Matt Cassel at the helm. Brady is signed through the 2017 season, so he’s locked in for at least another four seasons. He’ll be 40 when the contract ends. Polian said no quarterback has played well beyond 39, but his money would be on either Brady or Manning to change that statistic. “Father Time is undefeated, though,” he said with a smile. With that in mind, is Mallett willing to wait another three years, or more, before finally getting to steer the ship? His status is a major piece of the puzzle when planning for the future beyond Brady. The Pats haven’t drafted a quarterback since taking Mallett in the third round in 2011.