Well, rookie starter might be a stretch at this point. The Twins, suprisingly, held onto Lohse and offered him arbitration. That means the opening day rotation is most likely Santana, Radke, Silva, Lohse, and Baker. Terry Ryan has said they want Liriano getting his share of innings and that he won't start the season in the bullpen if he doesn't make the major league rotation... which would mean more seasoning in Triple-A. Not that he has much to prove at Triple-A anymore.
Baker projects to be a two-or-three starter, and is already a very solid fourth starter caliber pitcher. Very polished but his ceiling isn't much higher than his current value. He'll win the fifth spot out of spring training.
If Lohse gets dealt before opening day, however, then I would expect Liriano to start the season pitching in the bigs. Although, dark horses like Matt Guerrer or Boof Bonser might be used to shelter Liriano a bit longer.
Just for fun's sake, the conservative PECOTA pegs Liriano for a 3.87 ERA and 1.29 WHIP with 150 K and 57 BB in 158 IP. That projection hasn't taken into account how much the Twins will let Liriano pitch, but what he would likely produce as a starter from opening day onward.
As far as potential and ability, I think Liriano is as safe as pitching prospects come. His name was mentioned every time the Twins brought up trade talks for a hitter. A lot of different scouting departments love the kid. The cocern is if he can keep his mechanics stable and consistant and avoid any more shoulder problems. If he gets to the show, Rick Anderson is the most underrated pitching coach in baseball (moreover, simply one of the very best), so that's encouraging. If Anderson can coax a 3.44 ERA out of Carlos Silva and a 4.18 ERA out of Kyle Lohse you have to be excited about his chances with someone that can actually miss bats.