The tricky Irish winger hit the clinching goal for the U's in a 2-0 win against Rotherham that means they need just a draw at Yeovil next weekend to secure automatic promotion from League One to the Championship.
But they'll have to do it without Yeatesy. He was back at the Lane on Sunday to see the medical team after feeling a pain in his shoulder in the latter stages against Rotherham and it was bad news - he has dislocated his shoulder and needs surgery.
That news came as a huge blow to the 21-year-old after playing 44 of the U's 45 league games since his loan switch in August.
"I don't know what to say," said a downbeat Yeatesy. "I put my arm up to call for a cross and it felt like my shoulder just moved out of place.
"It hasn't quite sunk in yet, it's a bit of a freak injury but that's it for my season having played just about every game.
"I've been playing well, creating goals, scoring goals and I was buzzing after Saturday but this has knocked the wind out of my sails to be honest."
Mark Yeates came crashing down to earth after his goal moved his loan team Colchester United to within a point of promotion on Saturday.