Zebo cracked a bone in his foot during Munster's defeat to Edinburgh at Murrayfield on Saturday and will now be out of action for 10 weeks.
The 23-year-old limped out of the Round One encounter with seven minutes remaining and will miss Saturday's home tie with Gloucester and is likely to sit out the December double header with Perpignan.
"Simon Zebo has been ruled out of action for approximately 10 weeks following on from the injury he picked up in Saturday's opening Heineken Cup game against Edinburgh," read a statement from Munster.
"A scan has identified a crack in a bone in his left foot and Zebo will continue to wear the medical boot he was issued post-match for the next four weeks."
Allen faces a similar spell away from the game after he was ruled out for 12 weeks with an unusual leg injury that is more commonly associated with road traffic accidents.
The in-form centre suffered the setback in a domestic derby with Northampton Saints the week before last and has already missed Friday's trip to Ulster.
Allen, who was being tipped to challenge for an England Test spot in the absence of Brad Barritt and Manu Tuilagi this autumn, won't return in time for December's Heineken Cup games with Montpellier and could even remain unavailable for Rounds Five and Six when Leicester travel to Treviso before welcoming Ulster to Welford Road.
"It's a very unusual injury, one that normally only comes from road-traffic accidents, and thankfully our medical staff spotted it early enough and it has been dealt with," said Leicester's director of rugby Richard Cockerill, who is now without his first-choice midfield following Tuilagi's chest injury.
"It's very unfortunate for him: he just got a whack on his leg and a bruise, but the blood collected in the compartment of the muscle. The blood builds pressure into the muscle and constricts it and basically part of the muscle dies and will start to rot and turn into gangrene if you don't get it.
"The ramifications of it can be quite horrendous if you don't get it early. He had surgery - they opened him up to relieve the pressure, take all the dead stuff out of his leg and leave the wound open to heal from the inside. He will make a full recovery but it is just time now."
Toulon scrum-half Claassens is expected to be missing from his side's squad for the next six weeks after he suffered a broken hand during Sunday's 50-point win over Glasgow Warriors.
The former Springbok has made a mighty impression since joining the Heineken Cup holders from Bath during the summer but he was forced off in the 51-28 victory at the Stade Felix Mayol and will definitely miss Saturday's lunchtime kick off against the Blues in Cardiff.
Claassens should return in time for the back-to-back games against Exeter, though, with the French giants travelling to Sandy Park in Round Three on December 7 before hosting Rob Baxter and co on the Cote D'Azur exactly a week later.
His halfback partner, Jonny Wilkinson, also picked up an injury to his hand in the Glasgow game but, although the wound required stitches, England's World Cup winning hero should be fit for the trip to the Welsh capital in three days' time.
Northampton prop Alex Corbisiero isn't expected to feature in Round Two, however, with rumours of a suspected knee problem circulating in the English press.
The Saints are yet to confirm reports that the British & Irish Lions Test star experienced an issue during Saturday's 19-13 reverse in Castres but The Times has suggested that he could be unavailable for three to four weeks, starting with Sunday's crunch clash with the Ospreys at Franklin's Gardens.