Ospreys boss Scott Johnson admits Richard Hibbard has fitness issues after the hooker was left out of Wales' 35-man Six Nations training squad.
The 28-year-old, 17st 4lbs powerhouse was overlooked as Wales coach Warren Gatland picked Matthew Rees, Ospreys team-mate Huw Bennett and Ken Owens.
Hibbard missed playing for Wales at the Rugby World Cup due to an ankle injury.
"There's a side to Richard which we love and there's a side that lets him down," said Johnson.
"And he has got to understand there are physical requirements all of the time and he has to acknowledge that all of the time.
"To compete with opponents who are up against you all the time you have to got to make sure that you are physically at 100%.
"Richard sometimes can ebb and flow with regards that.
"As a rugby player he should be in the team. Physically, sometimes, he lets himself down.
"And he wouldn't mind me saying that because I'm on his back all of the time.
"I've got a soft spot for Rich as a kid. He epitomises everything I like."
He had been on course for a place in Wales' World Cup squad when he replaced neck injury victim Rees for Wales' final warm-up game against Argentina last August.
Hibbard, however, picked up the ankle injury in the second half and subsequently missed the trip to New Zealand - and he has been on the comeback trail ever since returning to play for Ospreys against Ulster in September.
Hibbard, who has only started four times for Wales in 15 appearances, is not the only Osprey to miss out on Gatland's squad that travels to the Spala ice chamber in Poland on Sunday.
Both utility forward Jonathan Thomas and Dan Biggar were left out of Wales' Six Nations campaign that begins against Wales v Ireland in Cardiff on Sunday, 5 February.
"JT is a versatile rugby player," added the Ospreys director of rugby.
"Has his form waned? Yes in comparison to others, but selfishly we are glad to keep him. I'm disappointed... because I still feel JT has a valuable role for Welsh international rugby. I do think that."
Johnson, who leaves the Ospreys at the end of the season to take up a senior coaching role with the Scotland, has been a long-time admirer of Biggar but he has fallen behind Rhys Priestland and James Hook, who has been picked as an out-and-out fly-half.
"I keep saying [Dan is] born in the wrong country," he added.
"I think a lot of issues the Welsh rugby public [have with him] is his persona on the pitch. Off the pitch he is nothing like that. That is the irony to it.
"I think we get caught up on his ability and I am disappointed for him that is not getting another chance to play there.
"I think they [Wales] are pretty strong there and I can see the logic in that but I am disappointed for the kid."