They absolutely would've been a serious contender for the championship with a healthy rondo and supporting cast, but yeah now the season is over. Hopefully they unload whatever they can for draft picks and start to re-build.
Boston wouldn't want Pau. They would want to rebuild with draft picks / expirings and young guys. Pau has another year after this with 20 mil left and Gay has north of 50 mil left on his deal.
Tough break for Boston. Rondo is a fierce competitor that plays the game the right way. No doubt woulda had a puncher's chance with him in the lineup for the playoffs