4. Diego Lugano
Current Club: Unattached Lugano might not be the most fleet-footed of defenders - which is probably why he now finds himself without a contract, having been released by West Brom this summer - but what he lacks in that department he makes up for in terms of his vision and his positional sense. The former Fenerbahce stalwart played only 9 times for the Baggies last year, but his wealth of experience would be a huge boost for Celtic, and frankly, even without pace he would be an improvement on Efe Ambrose at the minute. Lugano might cost a relatively large amount in wages - which is arguably why West Brom let him go midway through a two year deal - but he would undoubtedly press for a first team place easily at Celtic, and would be able to cope manfully with the rigours of the SPL, as long as he wasn't caught too much on the counter-attack, which would probably require something more in defensive midfield from his potential team-mates.