The All Blacks legend damaged his Achilles tendon in last weekend's 19-13 defeat to the Ospreys in the RaboDirect Pro12.
A Munster statement said: "Doug Howlett will have a scan and see a specialist early this week to determine the exact nature of the injury he sustained against Ospreys.
"Initial examination suggests he's damaged his Achilles tendon and he has definitely been ruled out of Munster's upcoming Heineken Cup matches against Scarlets."
Munster travel to Parc y Scarlets on Saturday, with both teams boasting 100 per cent records after the opening two rounds of games.
The Scarlets also have injury concerns regarding one of their wings, after George North limped off towards the end of Wales' defeat to Australia.
The Welsh region had five players in the match 22, while centre Jonathan Davies missed the game with a groin complaint.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
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Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
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The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
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The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.
