Typical recovery is 3-4 weeks before resuming a throwing program, putting an estimated return for the right-hander sometime in May at the earliest. McClellan underwent an MRI on Saturday, which showed no structural damage.
McClellan has been scratched from his scheduled start on Sunday, which would have been his second since missing nearly three weeks of camp with a sore right shoulder.
The injury is especially troubling as McClellan was one of many pitchers who are battling for the open fifth spot in the Rangers starting rotation. Between injuries and ineffectiveness, the number of quality candidates is quickly shrinking.
McClellan felt discomfort during his bullpen session on Friday and said he didn't like how he felt. He was examined by doctors on Saturday, when the the diagnosis was made.
The newcomer to the Rangers looked sharp in his Texas and spring debut Tuesday against the Milwaukee Brewers in his first game action since being sidelined last May.
With the exception of one meaningless walk, McClellan was perfect during his two scoreless innings. He showed no rust in his return from mid-season shoulder surgery and showed no signs of pitching in a game for the first time in 10 months.
"It was just a big relief for me to get the first one out (of the way)," McClellan said after the game. "You envision that first one going smoothly but you're not really sure. There's always in the back of your mind, 'am I going to get back to where I was?' and I felt I was right where I left off."
The 28-year old was signed by Texas as a free agent in January. McClellan played all of his previous five years in the majors with the St. Louis Cardinals. He missed most of the 2012 season with right shoulder and elbow injuries and had shoulder surgery last July. The soreness that sidelined during most of spring training was a byproduct of that procedure.