Ronaldo, 37, is heading to Qatar and will captain Fernando Santos' side in what may be his final appearance a World Cup tournament.
However, the United forward has struggled for form this season, scoring just three goals and contributing two assists in 16 appearances across competitions.
He didn't fare any better for the Portugal national side during their recent UEFA Nations League action in September.
The Red Devils striker didn't manage a single goal and offered just one assist in a 1-0 defeat to Spain and a 4-0 win over the Czech Republic.
Davis has reacted to Cristiano Ronaldo's inclusion with bemusement.
The Englishman can't understand why Portugal are taking a 37-year-old to the tournament in Qatar.
Davis also suggested that his Wolves midfielder Joao Moutinho will be disappointed by the veteran striker's call-up.
He told the media in his pre-match press conference ahead of Wolves' clash with Arsenal (via MARCA):
"I haven't spoken to Joao about it, he was very focused on the stuff we were doing for Arsenal tomorrow. He's very professional, I'm sure he'll reflect on not being selected and he will be disappointed with Ronaldo going at that age."
Davis believes that Santos was looking to balance his squad by picking the former Real Madrid forward:
"The coach has to think about the balance of the squad, it's important for any coach, so maybe that was part of the decision."
Cristiano Ronaldo has appeared at four World Cup tournaments - 2018, 2014, 2010, and 2006.
He has made 17 appearances, scoring seven goals and providing two assists.
Manchester United are interested in signing Brentford striker Ivan Toney as Cristiano Ronaldo's replacement
Cristiano Ronaldo's contract with Manchester United is set to expire next summer.
The Red Devils do possess a one-year extension option, but that seems unlikely to be taken up.
The Portuguese attacker was eager to leave the club this past summer and head to a side competing in the UEFA Champions League.
No legitimate bid was made for Ronaldo, and he remained at Old Trafford.
However, if the veteran forward does move on next summer, Manchester United are reportedly looking at Brentford striker Toney as his replacement.
The Englishman has been in superb form this season, scoring 11 goals in 16 appearances across competitions.
This includes a double against reigning champions Manchester City on 12 November as the Bees secured a shock 2-1 victory.
Toney has two years left on his contract at the Gtech Community Stadium, and he may cost around £45 million.