Eagleton had finished Friday's training session and was crouching down to get his breath back when his knee locked.
The 31-year-old had an arthroscopy later on Friday at The Avenue Hospital in Windsor, and Doggies medical service manager Andrew McKenzie told westernbulldogs.com.au that it happened in the most innocuous way.
"It was quite undramatic, really, how Eags locked his knee - he just had a little bit of cartilage jam up the joint, and unfortunately we needed to get that fished out by a surgeon," he said.
"So he had a little arthroscope, the knee was cleaned out - he removed a little bit of frayed cartilage.
"But Nathan's got his full range back, his full movement now, so we're not expecting him to be out for long, it should only be probably three weeks that it disrupts his pre-season for."
McKenzie said condition of the cartilage was such that the problem could have occurred at any stage of the year.
"If it had to happen, now is the best time for it, because he's got a good base fitness, we've got through the major part of our training loads, and he's only missing three weeks.
"He'll be able to condition fairly shortly, as long as we can get the wound and the swelling in the knee under control.
"He'll be up and going very quickly."
Nathan Eagleton averaged 80.32 points as a midfielder in last year’s Toyota AFL Dream Team
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