The deal, which came together in the last 48 hours, sends Napoli and cash considerations to the Rangers for a player to be named or cash to the Red Sox.
"He can play first base or designated hitter depending on how [manager Jeff Banister] wants to go against left-handers," Daniels said. "As far as an emergency third catcher, he had that role with the Red Sox and will here. We did not acquire him to come in here and catch."
Napoli was scratched from Boston's lineup late on Friday against Detroit. The Rangers hope Napoli can get to Seattle in time for Saturday's game against the Mariners. The Rangers are facing left-hander Mike Montgomery and they are 19-23 when facing a left-handed starter this season.
Napoli has played in 98 games for the Red Sox and has hit .207 with 13 home runs, 40 RBI and a .386 slugging percentage. He has a .229 average with a .500 slugging percentage against left-handed hitters.
Since the All-Star break, Napoli's hit .267 with three home runs, 10 RBI and a .533 slugging percentage in 18 games.
"Napoli is a guy who has always hit left-handers," Daniels said. "He has been very productive against them this season. That's something we have struggled with. He is a guy who we know will fit our culture with his makeup. There will be a minimum transition period. He gives us presence and power against left-handers. It made sense for us."
Daniels said he talked with the Red Sox earlier this season about Napoli. A deal was difficult because he is limited to first base and designated hitter, positions the Rangers already had covered. But with rosters expanding in September, Daniels said it became a more attractive option now.
Napoli was a big part of the Rangers success in 2011-12 when he was still primarily a catcher. He played in 221 games in those two years and hit .275 with 54 home runs, 131 RBI and a .552 slugging percentage. He started 126 games at catcher and 51 at first base.
He signed with the Red Sox as a free agent in 2013, but has played only at first base. He has not been behind the plate since he was with the Rangers.