But he couldn't. Apparently there is an obscure rule regarding it.

Shamelessly stolen from Miked Up (Mike Wilner's Blog)

Before I go, I learned something about the bureaucracy of baseball today, a little-known rule about which I wasn’t aware  was applied in the case of Dirk Hayhurst, who was called up last night.  It turns out that the Jays weren’t allowed to call him up from the minors until May 15th, so he couldn’t have made the team out of Spring Training regardless.   Cito Gaston said before the game that he had actually wanted to take Hayhurst north because of his fantastic spring (8 IP, 7 hits, 1 run, 0 BB, 13K).

The reason he couldn’t was that Hayhurst was a "draft-excluded" player who had been released after the Rule 5 draft.  That is, a player who was added to the 40-man roster after X date (sometime in August, I think).  Such players aren’t allowed to be placed on the major-league roster until May 15th if they re-sign with the team that released them.  We got all the info in a nice, little Baseball 101 session - though given the intricacy and obscurity of the rule, maybe Baseball 401 - with Alex Anthopoulos before the game.  Basically, the rule is in place to prevent teams from hiding players from the Rule 5 draft, so it bites teams that kind of try to make their way around said draft.   That’s not what the Jays were trying to do here, Hayhurst was released and quickly re-signed in February after the Jays had a chance to grab Matt Bush on waivers.

It doesn’t happen often, that a team has a player on its 40-man roster through the Rule 5 draft, then releases him and re-signs him prior to the start of the regular season (such players can’t be outrighted off the 40-man, the only way to get them off the roster is to release them), but when it does happen, we now know that those players can’t play in the big leagues until after May 15th of that season.  You learn something new every day!

You almost need a degree to figure out half the rules that Baseball has....