No one claimed Liriano was a consistently great pitcher like Greinke or Hamels. He is still, overall, an above average pitcher, and lefthanded, and comparable in objective value (WAR) to Sanchez (Liriano has actually been more valuable since 2006).
The haul is disproportional, and it signals that Jr. mishandled the situation. The difference with Milwaukee, is that the Brewers were not selling Greinke so much as trying to re-sign him. With Florida, they had, and are having a firesale. The Twins are putting Span, Willingham, Morneau on ebay with an unrealistically high minimum researve. But Liriano they seemingly dumped on the curb and slapped a cardboard "4 sale" sign on him.
This from the team whose biggest clearest weakness is starting pitching.
To negotiate effectively you have to be willing to walk away, and instead Jr's FO did everything to indicate that the Twins were not going into August with Liriano on the staff.