Jacques Nienaber says Leinster will know more about the extent of Caelan Doris' shoulder injury later in the week but the coach believes it could be "quite serious".
The Leinster skipper was replaced in the second half of his side's Investec Champions Cup semi-final loss to Northampton on Saturday and has been ruled out of this weekend's URC game against Zebre.
The 27-year-old is now is a fitness battle to make the Lions tour. Andy Farrell will name his squad on Thursday and Doris was among those linked with the captaincy.
"He saw a specialist and I think there's some further investigation that needs to be done," Nienaber told RTÉ Sport.
"Fingers crossed that things aren’t as bad as we think it is.
"But in the last couple of months that I’ve worked with Caelan, he’s not a guy that comes off the pitch with injury so when I subjectively saw him coming off the pitch on Saturday, in my mind I thought this must be quite serious.
"We’ll probably know more at the back end of the week but my gut, and it’s only my gut, says that I think it might be serious, but fingers crossed it might not be."
The Lions begin their three-match Test series against the Wallabies on 19 July in Brisbane before playing in Melbourne a week later and finishing in Sydney on 2 August.
Nienaber added: "It’s difficult for me to comment on the selection but it will definitely be a race.
"Like I say, a shoulder injury depends on the structure, the damage, it can be anything between six weeks to two months versus a four to six-month thing so depending on the amount of damage within the shoulder joint and all the structures involved.
"That will determine that. We’re not 100% sure, we know there’s damage.
"Caelan, as a person, I don’t think I’ve ever seen him leave the pitch with an injury that wasn’t serious."
Ireland lock James Ryan and winger Rob Russell have returned to full training for the province.
Watch Leinster v Zebre in the URC on Saturday from 5.10pm on the RTÉ News Channel and RTÉ Player. Listen to updates on Saturday Sport on RTÉ Radio 1
