Mavs have enough wing players, they need a defensive stopper under the rim to help Lively. Trading a much needed scoring combo guard for another wing does nothing for the team.
Rui is 6'8" with a 7'2" wingspan and athletic which is exactly what Lively needs . Presently Grant Williams under 6'6" has a short wingspan of 6'9 3/4" and very unathletic.
Watch the games and look how small the lineup looks with THJ and watch how many points are scored against the Mavs when he plays. He's not a plus when he plays.
The Nuggets play with a athletic high flier in 6'8"Aaron Gordon and the 6'10" MPJ. AG has a 6'11" wingspan/MPJ has a 7'0 wingspan so Rui is big enough to play with these guys and is an offensive threat that they have to guard him back.
Having Lively's 7'8" wingspan and Rui's 7'2 wingspan is a good start to matchup with the Denver team.