Zambrano missed most of 2006 after having Tommy John surgery in May.
Zambrano, when healthy, has been a league average pitcher - his career ERA+ is 99. However, even before his injury Zambrano issued a lot of walks. Coming off of TJ surgery that will be one of the areas to focus on.
This sounds like a double win here: One, Zambrano is signed to a minor league deal so there's no real cost unless Zambrano can contribute this year. Two, the Jays have a team option for 2008 so if Zambrano recovers and pitches well the Jays will have another pitcher under their control. Frankly, I'm surprised Zambrano would agree to this unless it's a substantial team option.
Zambrano won't be 100 per cent at spring training. But he could be ready by mid-season, perhaps able to give the Jays a boost should one of their starters struggle or get injured. Said general manager J.P. Ricciardi, "There's still some rehab that needs to go on but we sent someone to watch him throw in Venezuela and it looks like he's coming back."
This is just speculation on my part, but I suspect that the Jays will leave Zambrano behind in Dunedin (where it'll be warmer than Syracuse or NH) and he'll work there for a bit. If he pitches well he could become another option for the Jays.