FDA guidance is: Advantages of fast track designation include...

FDA guidance is:

Advantages of fast track designation include actions to facilitate development and expedite review of the product, such as the possibility for rolling review if FDA determines, after preliminary evaluation of clinical data submitted by a sponsor, that the fast track product may be effective. In addition, such a product could be eligible for priority review if supported by clinical data at the time of marketing application submission.
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A decision about granting priority review is made within 60 calendar days of receipt of the marketing application or efficacy supplement. If priority review is granted, CBER has a 6 month goal for reviewing the biologics license application (BLA) or efficacy supplement.
https://www.fda.gov/media/120267/download

If approval requires another trial, and the endpoint is pain reduction at 12 months, then it looks like the timeframe for approval may be as soon as ~18 months after the trial completes adequate recruitment and data validation. Perhaps a trial design would include Bayesian analysis with interim reviews to fast-track the recruitment. If such a trial design were used, MSB may have a justification for an 'informative prior' from the excellent results of the previous trial.
https://en.wikipedia.org/wiki/Prior_probability#:~:text=In%20Bayesian%20statistics%2C%20Bayes'%20rule,uncertain%20quantity%20given%20new%20data.

Interesting wording in the statement from SI:
"We look forward to working closely with FDA to efficiently generate the additional data needed to support marketing approval of rexlemestrocel L".