* Button qualifies ninth but drops to last
* Penalty imposed after power unit part replaced
* Briton is standing in for absent Alonso (Adds detail after qualifying)
By Alan Baldwin
MONACO, May 27 (Reuters) - Jenson Button will start Sunday's Monaco Grand Prix, possibly his final Formula One race, from last place after McLaren's engine partners Honda detected a power unit problem that incurred a 15-place grid penalty.
Button, the 2009 world champion and a former winner in Monaco, qualified ninth but his sterling efforts were ultimately in vain.
The 37-year-old Briton is standing in for Fernando Alonso in the Mediterranean principality while the Spaniard competes in the Indianapolis 500 on the same day.
The team's ongoing engine problems will only confirm to Alonso, who has been the centre of attention at Indianapolis, that he was right to miss Monaco for a shot at glory across the Atlantic.
McLaren are the only team yet to score a point this season, the former champions beset by a lack of reliability and performance, but anything can happen in Monaco and Button was not downcast.
"A lovely experience, lots of new memories of driving this type of monster of a car," he said. "It's probably the quickest McLaren that has ever driven around here, in terms of lap time.
"It's tricky but I've loved it and enjoyed working with the engineers.
"Tomorrow is obviously a different day but today's the day that I knew was going to be the fun day," added the Briton. "I definitely extracted a lot of fun out of today."
McLaren Racing Director Eric Boullier said Button had done a "perfect" job.
"He was ready, fit, fast, smiling. So -- perfect."
Monaco had represented a real chance for the team to change that, however, with the tight and twisty street circuit rewarding driver skills and making engine power less of a factor.
Button, who had not driven a Formula One car for more than six months until he stepped into the cockpit for Thursday practice, had been 12th in final practice.
Belgian team mate Stoffel Vandoorne, who has a three-place penalty carried over from the previous race in Spain, qualified 10th and drops to 12th.
Editing by Pritha Sarkar