Mike Singletary has come to the same conclusion that coaches in Houston, Carolina and New York came to before him: that he’s seen enough of David Carr to know that he can’t be counted on as a starter.

Alex Smith is expected to miss 2-3 weeks with a shoulder injury and while some 49er fans want to see Carr take the snaps this Sunday in London, Singletary has already named Troy Smith the starter.

Hey, why not?

Look, Carr is nothing more than a backup and will be nothing more than a backup. The same could be said about Smith, but the former Heisman Trophy winner has one thing Carr doesn’t: upside.

Smith’s height is a major issue, but his athletic ability makes him intriguing and he has a strong enough arm to make all the throws at this level. Seeing as how he’s been running the scout team offense in practice, it would be wise if offensive coordinator Mike Johnson built his game plan around Frank Gore this week and limited Smith’s throws to screens, roll outs and three-step drops. That said, once Smith gets more comfortable with the offense, there’s no reason to think Johnson can’t expand the playbook.

There’s an argument to be made that Singletary didn’t give Carr a fair look, but what more does he need to see? What more does anyone need to see out of Carr to know that he can’t run an offense? When the Texans finally released him, everyone thought that with a good offensive line he would turn his career around. But he had a decent O-line in Carolina and he struggled. The Giants, who have had been searching for a backup for Eli Manning before trading for Sage Rosenfels, also didn’t see the need to keep Carr around.

He is what he is, which is why Smith is worth the risk for the 49ers. Will it ultimately be the right decision by Singletary? Who knows – time will only tell. But at least Smith’s ceiling hasn’t been met, unlike Carr’s.