Nelson wouldn't be able to accrue a full season of service time if he was called up today. He's only currently listed (at Baseball-Reference.com) as having 27 service days, and by my count he wouldn't be able to notch more than ~115 or so service days the rest of this regular season. For anyone unfamiliar, a full year of service time is defined as 172 days (with a full MLB regular season typically having around 180-185 service days) There is a possibility of him becoming a Super Two player (meaning, eligible for arbitration before notching a full three seasons' worth of service time), but I'm not sure how well that can be predicted this far out. This MLBTR piece cites the following: "As MLBTR's Tim Dierkes wrote in October, qualification [for Super Two status] has landed between 2.122 and 2.146 years of service in recent years." If Nelson were called up tomorrow & stayed up the rest of the way, he'd finish with 142 service days by my count. So it could well be that the Brewers are waiting to call him up until he'd likely be safe from earning Super Two status. If this is the motivation, I wouldn't expect to see him up (barring an injury in the rotation) until July.