The only question is not whether you sign-- if you can, you do-- it's whose roster spot he gets. (I vote Cassell, but I suspect it would be O'Bryant.)
His age is a concern, but look at what he did in the regular season last year, compared to what P.J. Brown (Brown's numbers in parentheses). All numbers per 36 minutes (though it's worth noting that Mutombo played more mpg than Brown, 16 vs. 11).
FG: 2.5 (2.4)
FG%: .538 (.341!)
RB: 11.6 (11.7)
AST: .3 (1.7)
BLK: 2.8 (1.4)
PTS: 6.9 (6.7)
So Brown had more assists and slightly more rebounds, Mutombo had more of everything else. Mutombo is also listed at 7'2'', while Brown is listed at 6'11''. Also, Brown is supposedly fully retired, while Mutombo publicly wants to come back and has never won a ring.
Get it done, Danny. And make Cassell a full-time coach while you're at it. Or maybe cut Scal.