The 24-year-old has appeared more than 50 times for Cardiff and captained the Wales under-20 side at the 2013 Junior World Cup.
And he made his debut for the senior side last year but will miss out on the autumn internationals after suffering a brutal hamstring tear.
“There are three hamstrings, as I understand it, and he tore one of them off the bone,” Cardiff Blues coach Danny Wilson said.
“They re-attached that part. It wasn’t as bad as the Dan Fish one last year where all three were torn off the bone.
“Ellis’ was just one and that’s been repaired and he’s three to four months from playing again.
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“We are probably not going to see him back until the new year.”
The injury to Jenkins means Blues are without two key men in the back row as Sam Warburton takes time off after a brutal British & Irish Lions tour of New Zealand.
However, Wilson will resist the temptation to bring the experienced openside back earlier than planned.
“He had an extremely long summer and was involved in a lot of high profile rugby,” Wilson added.
“It’s no different to Jon Davies, Alun Wyn Jones and people like that.
“They are all pretty much in the same boat.
“Each player comes back into pre-season from a break and has to do a certain volume of running and rugby work to not be an injury risk themselves going into a high intensity game.
“Sam needs to get a certain amount of training under his belt. He has done some of that.
“The most important steps are to make sure long-term player welfare comes first.
“Players are looked after both by us and the WRU and for the right reasons.
“The WRU have the overriding say with Sam because they pay the overriding money.
“We are involved in the process and have good dialogue and we all know where we are and what we are doing.
“Sam is in the middle of getting the volume he needs.
“When he is ready we will have him on the pitch. Hopefully that will be in the next week or two.”