“If they can link up with us again for the last week, then great, but I can’t see it if I’m honest.”
Townsend is scheduled to name his team to face Wales on Thursday, with the squad currently training in Spain.
“We’re fairly happy with the group we’ve got,” Horne said. “We’ve got cover there. I can’t see us adding anyone.”
Matt Fagerson, who started in the opening loss in Italy, took over from Ritchie against England, with Max Williamson replacing Dempsey.
Edinburgh winger Darcy Graham, who has scored four tries against Wales, also came off the bench at Murrayfield.
Dobie’s absence could mean a return for at least one of Duhan van der Merwe or Blair Kinghorn, the big-name British and Irish Lions backs who were not in the matchday 23s for the opening two matches.
Kinghorn is back with the Scotland squad after being released to play for Toulouse in Sunday’s 13-9 win over Stade Francais in France’s Top 14.
“The competition’s been really high in that area,” Horne said. “The three guys went well against Italy and they went well again against England, but Blair is a really supremely talented player and played well for his club.”
Scotland are aiming for a fourth successive victory over Wales, who have suffered big defeats to England and France in this year’s tournament.
However, Horne is aware of the statistic that, of the seven times Scotland have had a Six Nations match after beating England, they have lost six of them.
“We don’t want to be the same old Scotland,” he said. “They are all aware of the challenge.
“Wales will be in a very similar position to us last week. They are a wounded team at the moment and will have that desperation that we had on show last week.”
Wales now have former Scotland defence coach Steve Tandy in charge, with former Scotland forwards coach Danny Wilson as assistant.
“We know how good a coach Steve is,” Horne added. “Danny as well is a brilliant coach and they will be desperate to beat us.
“Steve is a very good defence coach and that is another motivating factor – we know how good our attack is going to have to be to break them down.
“But we have just got to focus on ourselves and make sure we do a good job.”
