The All England Lawn Tennis Club (AELTC) barred players from the two countries from competing at this year's grasscourt major in response to what Russia calls a "special military operation" in Ukraine. Belarus has been a key staging area for the invasion.
"I think it's very unfair to my Russian tennis mates, my colleagues. In that sense it's not their fault what's happening in this moment with the war," 21-times major winner Nadal told reporters at the Madrid Open on Sunday.
"Let's see what happens in the next weeks, if the players will take some kind of decision ... there's one thing that's negative, there are things that are clear. When the government imposes some restrictions, you just have to follow them."
AELTC Chairman Ian Hewitt said British government guidance did not allow players to compete at the event based on their rankings and there were two available options -- declining entries or allowing entries but only with specific written declarations from individual players.
Britain's former world number one Andy Murray said there was no right answer on the issue.
"I'm not sure how comfortable I'd feel if something happened to one of the players or their families (as a result of signing the form)," Murray said.
"I don't think there's a right answer. I've spoken to some of the Russian players... some of the Ukrainian players.
"I feel really bad for the players who aren't allowed to play and I get that it will seem unfair to them. But I also know some of the people who work at Wimbledon, and I know how difficult a position they were in."
The AELTC's decision has been condemned by both the men's and women's tours as well as several other players.
World number one Novak Djokovic, who has also criticised the ban, said on Sunday that he had spoken to Russian players during last week's Serbia Open and it was tough for them to be excluded from the tournament.
"It's hard. I understand that there is frustration. ATP is going to, I guess, analyse the whole situation and understand what can be done," said the Serbian.
"I stand by my position that I don't support the decision. I think it's just not fair, it's not right... now I guess it's on player council, the tour management, to really decide along with the players what is the best solution in this situation."