There was definitely an operational MER during Apollo 13. Throughout the mission on most of the loops - the first 2/3 of the Kranz shift just before the accident is a good example - the controllers make reference to Building 45 and Don Arabian. Bldg. 45 was the location of the MER, and Don Arabian was its leader. During that shift, he and the SPAN bigwigs (Joe Roach and a few others) talked about how they were going to bleed down the SHe (supercritical helium) if the telemetry showed excessively high readings during the LM pass.
The problem apparently was so critical that when Kranz called up SPAN to let them know he was powering up the LM early, there was nobody available in SPAN to give him either a go or no-go. The controller who answered him, Bill Blair, was a North American contractor. 😅
Link:
https://apolloinrealtime.org/13/?t=053:22:11&ch=50During and after the accident, the MER spread out to encompass nearly the entire Manned Spacecraft Center as they vacuumed up controllers and contractors, working furiously to develop procedures and solve imminent problems regarding the spacecraft. John Wegener, who was the CONTROL during one of the Griffin shifts during translunar coast, is never seen or heard from again after that shift. It's plausible to guess he was conscripted for MER duty, and I'm sure there were others.