At 48, the one-time adland poster boy decided he still had sufficient vigour to take part in the relay race at the agency's sports day.
However, just a few feet from the finishing line, he pulled a hamstring. Not only did it hurt like hell, but it was a tad inconvenient given that he was flying to Los Angeles on group business the following day.
Not that MacLennan's spot of bother got him much sympathy from the spectators. "He claims it was a hamstring but we all thought it was just a pulled muscle," one says.
"Probably the only thing that really hurt was his ego because he didn't win."
Now it has to be said that pulled hamstrings aren't to be taken lightly. Athletes are advised to rest immediately, apply ice packs and compressive bandages to control the swelling if necessary and find some crutches if walking is painful.
MacLennan, though, vowed to live up to the "nothing is impossible" Saatchi mantra and did not postpone his West Coast trip. However, because of his mobility problems, he asked his PA to phone ahead and warn the hotel in LA that he might need a bit of help on arrival.
When the hotel's doctor got to hear of this, he was aghast. What exactly did MacLennan think he was doing flying in such a condition, he wanted to know.
The result: MacLennan was rushed from the plane straight to hospital where his two hours of treatment included some uncomfortable blood-thinning injections in his stomach.
Then he got the bill. And that's when the real pain began.