The open-side suffered a fracture to his hand in Edinburgh's Champions Cup win over Montpellier last Friday.

Scotland open their campaign with home games against Italy on 2 February and Ireland a week later.

"To lose someone like Hamish is disappointing," said Townsend, who hopes Watson may still feature.

"But it is an opportunity for someone else."

Newcastle's John Hardie and Gary Graham are other open-side options, with blind-side Jamie Ritchie able to play on both flanks.

Watson, 27, has started 11 of Scotland's last 12 Tests and been in superb form again this season, helping Edinburgh into the quarter-finals in Europe.

He joins a casualty list that now numbers 20 in all, including former captain John Barclay, who is set to miss the whole tournament, and fellow back-rowers Blade Thomson, David Denton and Matt Fagerson.

Townsend said he expects "six or seven players" to return to action after their second game against Ireland, a group which includes Glasgow front-rowers Fraser Brown, George Turner and Zander Fagerson.

"Whether they are available to us or in the right condition to play our third game against France [on 23 February], it will be a boost to know these players are back for at least two, maybe three games," he told BBC Sport.

'We are in it to win it'

Scotland equalled their best-ever Six Nations finish of third last year in Townsend's first campaign as head coach, culminating in an unconvincing 29-27 victory over Italy in Rome.

They open this year's championship against the same opponents, who have not won a Six Nations match since beating Scotland at Murrayfield in 2015, one of seven occasions they have overcome the Scots in the tournament.

"The expectations in Scotland will be high, but we realise that it is a very tough game to start with," added Townsend.

"We have lost a number of times to Italy and we should have lost to them last year. They are a very tough team to play against, and Scotland teams in the past have found it very difficult.

"We are in it to win it [the Six Nations], and I imagine that is what every team is focused on.

"Finding that consistency of good-to-very-good performances is what is required from us to be tough to beat, and to win tournaments."

Italy have lost their last 17 Six Nations matches, 10 of them since Irishman Conor O'Shea took charge.

"We are not stupid," O'Shea said as he assessed their prospects for this year. "We know the scale of the challenge. We want to deliver our best and then we will see.

"Our only focus is delivering a massively intense performance, and to execute it against Scotland.

"We want to look back on a performance that we are proud of, and leave everything on the pitch."