Sri Lanka Cricket confirmed Nissanka's withdrawal on October 27, following an unsuccessful fitness assessment ahead of the tour.
The 15-man squad originally included him on the condition that he regained full fitness and obtained medical clearance before departure.
Udara, who has played five Tests for his country, was already in Pakistan as part of Sri Lanka's ODI tri-series contingent.
He had initially joined that squad as cover for Nissanka, who was also unable to satisfy the fitness requirements for the limited-overs competition. His availability has therefore provided Sri Lanka with a convenient replacement.
The injury dates back to Sri Lanka's white-ball tour of England in September and has since prevented Nissanka from featuring in T20I matches against Pakistan and the ongoing ODI tri-series.
He also missed the home Test series against India in August because of a wrist problem, extending his absence across several international commitments.
Udara brings previous experience in the longest format, having featured in Sri Lanka's two latest Test assignments against West Indies and India. His standout performance came in Antigua, where he scored an impressive 188 against the West Indies.
Sri Lanka will face Pakistan in a series forming part of the current World Test Championship cycle. The opening Test is scheduled for Rawalpindi from November 9 to 13, with the second match taking place in Lahore from November 17 to 21.
Sri Lanka booked their place in the ODI tri-series final with a commanding eight-wicket victory over Pakistan on Tuesday, eliminating England from contention ahead of Saturday’s decider.
Pakistan mounted a scarcely believable comeback to edge England by one wicket in an extraordinary ODI in Lahore on Sunday.
Australia all-rounder Cameron Green has claimed to be fit to play ahead of the third Test against South Africa at Newlands Stadium next week.
Sri Lanka executed one of the greatest run chases in ODI cricket, overcoming England's mammoth 430-run target in Rawalpindi to secure a remarkable victory.
